tag:www.mindcipher.com,2005:/feedMindCipher Puzzles2014-04-01T15:48:20Ztag:www.mindcipher.com,2005:Puzzle/1532014-04-01T15:48:20Z2014-05-29T02:00:09Z/puzzles/153What is the sum of 1+2+3+4+5 . . . (forever)?What is the sum of 1 + 2 + 3 + 4 . . . all the way to infinity?2014-04-01T15:48:20Ztag:www.mindcipher.com,2005:Puzzle/1522014-01-11T01:01:40Z2014-04-06T16:24:32Z/puzzles/152Lab Rat on the RunConsider a lab rat in a maze. There are four paths that can be taken with equal probability. Path A leads to exit in 5 minutes. Path B leads back to start and takes 8 minutes. Path C leads to exit in 3 minutes and last path D is 2 minutes long and leads back to start. What is the expected escape time of this poor rat from the maze? 2014-01-11T01:01:40Ztag:www.mindcipher.com,2005:Puzzle/1512013-12-25T19:19:50Z2014-08-19T22:34:59Z/puzzles/151Black and White HatsThere are 100 prisoners in jail. Their guards decide to play a game with them. Here's what happens:
1) The prisoners are lined up in a single file, front to back, such that the prisoner at the back of the line can see the 99 prisoners ahead of him, the prisoner immediately ahead of him can see the 98 prisoners in front of him and so on.
2) A randomly selected hat is placed on each prisoner's head. The hat may be black or white.
3) Starting from the **back** of the line (i.e. starting from the prisoner that can see 99 prisoners in front of him), a guard asks each prisoner what the colour of their hat is. If the prisoner answers correctly, he lives, otherwise, he is killed.
The prisoners are given these rules and then given 15 minutes to strategize before the game begins. What is the strategy that maximizes the expected value of the number of them that survive?
FAQ:
* **Can the prisoners take off their hats and look at them?**
No.
* **Do we know in advance how many of each colour hat there is?** No.
* **Can the prisoners communicate with the other prisoners in any way apart from responding to the guard's question after the strategizing time is over?** No.
* **Is an expected value of 75 survivors the best that can be achieved?** Nope :)
2013-12-25T19:19:50Ztag:www.mindcipher.com,2005:Puzzle/1502013-10-14T23:52:57Z2013-12-06T14:40:50Z/puzzles/150Cut the pizza into 11 equal slices with a wrist watchTools:
(1) A wrist watch and (2) a normal knife
Goal:
Slice a pizza into 11 equal pieces2013-10-14T23:52:57Ztag:www.mindcipher.com,2005:Puzzle/1492013-10-14T23:34:39Z2013-12-31T10:09:22Z/puzzles/149How far can you go?You have a 4 wheels car and a spare tire in the trunk.
The tire being used will be worn off for 1/4 part of it per mile (completely wear out for 4 miles)
How to make use of the spare tire to go as far as possible? 2013-10-14T23:34:39Ztag:www.mindcipher.com,2005:Puzzle/1482013-10-10T14:25:21Z2014-06-11T07:59:09Z/puzzles/148Binary 5 digitsWhat is the base 10 equivalent of the highest 5 digit binary number?2013-10-10T14:25:21Ztag:www.mindcipher.com,2005:Puzzle/1472013-08-28T10:48:22Z2014-08-16T10:14:46Z/puzzles/147SurvivorThere are 1500 people sitting around a circular table. They are numbered in a regular manner (1 to 1500 along the table).
Now, the 1st person gets a sword and kills the 2nd person. He then gives the sword to 3rd person, who then kills the 4th person and gives the sword to the 5th. This continues so that 1499th person kills the 1500th person and gives the sword back to 1st person.
Now the 1st person kills the 3rd and gives the sword to 5th person. This process continues until only one person remains. Which person remains?2013-08-28T10:48:22Ztag:www.mindcipher.com,2005:Puzzle/1462013-05-12T18:03:41Z2014-08-24T13:45:33Z/puzzles/146Charlie and the Chocolate FactoryCharlie once visited chocolate factory to buy some chocolates.Chocolate cost Rs. 3 each and he had only Rs. 45.He was disappointed. He wanted more.
But when he reached the factory he was amazed. There was a scheme on that special day. If you return 3 wrappers of the chocolate you will get one chocolate for free.
So how many chocolates can he buy?2013-05-12T18:03:41Ztag:www.mindcipher.com,2005:Puzzle/1452013-05-02T19:15:16Z2013-10-01T13:18:37Z/puzzles/1451978The year 1978 is such that the sum of the first two digits and the latter two digits is equal to the middle two digits, i.e. 19 + 78 = 97. What is the next year (after 1978) for which this is true?2013-05-02T19:15:16Ztag:www.mindcipher.com,2005:Puzzle/1442013-04-26T14:34:10Z2014-02-22T20:35:12Z/puzzles/144MatchsticksThis is a classic Martin Gardner puzzle.
See the picture - the four matchsticks represent a martini glass and the match head represents an olive. Move just two matches so that the glass is reformed but the olive is outside of the glass. The olive has to stay where it is. The glass may be turned to the left or right or be upside down at the finish.
The upper right diagram is wrong because the olive is still in the glass. The lower right diagram is wrong because three matches were moved. 2013-04-26T14:34:10Ztag:www.mindcipher.com,2005:Puzzle/1432013-04-21T20:36:22Z2013-11-26T20:31:18Z/puzzles/143Count The BitsWrite a function that counts the number of bits in an integer that are set to `1` that is linear in time complexity in the worst case, but sub-linear on average.2013-04-21T20:36:22Ztag:www.mindcipher.com,2005:Puzzle/1422013-04-21T05:45:38Z2013-08-14T16:25:39Z/puzzles/142Partitioning a setIn how many ways can a **set** of r objects be partitioned into **subsets** of k objects such that:
* all subsets are mutually exclusive
* each of k-object subset appears in exactly one of the partitions [[Partition wiki]](http://en.wikipedia.org/wiki/Partition_of_a_set)
given that k divides r.
Possible Applications: [Baranyai's Theorem](http://en.wikipedia.org/wiki/Baranyai's_theorem)2013-04-21T05:45:38Ztag:www.mindcipher.com,2005:Puzzle/1412013-04-13T21:31:01Z2014-08-19T22:35:50Z/puzzles/141Odd PillsYour pharmacist gives you 5 jars filled with pills. The pills contained in each jar have an identical appearance and size (i.e. if you mixed all of the pills you wouldn't be able to tell them apart).
4 of the 5 jars hold pills that weigh 10 grams each, and the remaining jar holds pills that weigh 9 grams each. However, you don't know which jar is the one holding the lightweight pills. Keep in mind that all of the pills in all jars are identical in appearance and size.
You are given a weighing scale that can only be used **once**. Using this scale (only once), **how do you determine which jar is holding the 9 gram pills?**
*Note: assume the jars are quite large and that you have an unlimited quantity of pills from each jar.*2013-04-13T21:31:01Ztag:www.mindcipher.com,2005:Puzzle/1402013-04-13T19:11:14Z2014-08-19T22:31:08Z/puzzles/140Multiples of 8 plus 1Squaring the first several odd numbers reveals the following pattern:
3 squared = 8 + 1
5 squared = 24 + 1
7 squared = 48 + 1
8, 24, and 48 are all multiples of 8. Does this pattern hold for all squares of odd numbers? Prove it!2013-04-13T19:11:14Ztag:www.mindcipher.com,2005:Puzzle/1392013-04-12T23:58:14Z2013-05-01T09:31:13Z/puzzles/139Grey Squares IllusionSquare **A** is the exact same shade of gray as square **B**.
It's true, check out *[the reconstruction](http://upload.wikimedia.org/wikipedia/commons/9/93/Optical_illusion_greysquares.gif)*.2013-04-12T23:58:14Ztag:www.mindcipher.com,2005:Puzzle/1382013-04-12T00:15:39Z2013-11-26T20:18:51Z/puzzles/138Non-Adaptive 12 Identical BallsOkay - so I posted [this puzzle](http://www.mindcipher.com/puzzles/28) to reddit a few weeks ago and one guy had the [following comment](http://www.reddit.com/r/programming/comments/17car1/i_was_asked_this_hard_interview_question/c84n65y):
*Same problem, but your solution must be non-adaptive. IOW, specify the weighings that you will make up front. You are not allowed to change them based on the results of previous weighings*
So the whole problem becomes: Identify the odd (heavy or light) ball among 12 identical balls **with a static plan (i.e. no conditional decisions)** that uses at most 3 weighings of a balance scale.
The original puzzle is hard enough, but this variant has been killing me. I think I could come up with a solution if the original problem specified whether we expect the odd ball to be heavy or light.
Anyone have any ideas? It feels nearly impossible.
2013-04-12T00:15:39Ztag:www.mindcipher.com,2005:Puzzle/1362013-03-21T23:03:37Z2014-03-13T08:23:38Z/puzzles/136Sum of Two Prime NumbersIf p, q > 2 are consecutive in set of primes. Since p,q can only be odd number, (p+q) is an even number.
Can (p+q)/2 be prime?
from http://republicofmath.com/2013-03-21T23:03:37Ztag:www.mindcipher.com,2005:Puzzle/1352013-03-20T19:24:53Z2014-08-19T22:30:39Z/puzzles/135Gold or silver?What weighs more, a pound of silver or a pound of gold?2013-03-20T19:24:53Ztag:www.mindcipher.com,2005:Puzzle/1342013-03-01T01:46:00Z2014-07-27T13:50:52Z/puzzles/134No if() Only abs()Given two variable a and b, construct a function f(a,b) which return the maximum one in {a,b}.
You can only use +- · / and abs() (to take the absolute value). NO if() is allowed in your construction.
Pure arithmetic ; )2013-03-01T01:46:00Ztag:www.mindcipher.com,2005:Puzzle/1332013-02-26T20:32:19Z2014-08-18T22:23:57Z/puzzles/133Swap Variables in Place without XORSuppose you have two variables, a and b. How can you swap their values without using a temporary variable and also without using the bitwise operator XOR?2013-02-26T20:32:19Ztag:www.mindcipher.com,2005:Puzzle/1322013-02-18T18:52:37Z2013-10-08T12:36:30Z/puzzles/132The Monty Hall ProblemYou’re on a game show, and you’re given the choice of three doors: Behind one door is a car and behind the other two doors are goats.
You pick a door, say No. 1, and the host, who knows what’s behind the doors, opens another door, say No. 3, which has a goat. He then says to you, “Do you want to pick door No. 2?”
**Is it to your advantage to switch your choice?**
This problem has numerous variations and its various forms have stumped many mathematicians.
Source: [Monty Hall problem - Wikipedia](http://en.wikipedia.org/wiki/Monty_Hall_problem)2013-02-18T18:52:37Ztag:www.mindcipher.com,2005:Puzzle/1312013-02-17T22:47:15Z2013-08-28T20:49:48Z/puzzles/131Two TrainsTwo trains (Train A and B) with 200 km between them are travelling towards each other at 100km/h. A mosquito, initially standing on the front of Train A, starts travelling towards Train B at 150km/h. Once it reaches Train B, it turns around and travels towards Train A. The mosquito continues going back and forth between Train A and B until they collide.
**How far does the mosquito travel before getting squashed between the two trains when the collide?**2013-02-17T22:47:15Ztag:www.mindcipher.com,2005:Puzzle/1302013-02-09T17:54:02Z2014-08-19T06:18:02Z/puzzles/130The Sinking IslandYou and 3 friends are on an island that is sinking. There is one bridge from the island to the mainland that supports *at most* 2 people at once. However, it's dark outside so anyone crossing the bridge must use a flashlight otherwise they'll fall off the bridge. Between the 4 of you, there is only one flashlight.
Let's name each of you Friend A, B, C, and D. Each of you travel at different speeds. It takes the following times for each of you to cross the bridge:
* Friend A: 1 minute
* Friend B: 2 minutes
* Friend C: 5 minutes
* Friend D: 10 minutes
When two friends cross the bridge, their rate is limited by the slower person. For example, if Friend C and D are crossing at the same time, it will take 10 minutes.
Friends can go back and fourth on the bridge. For example, Friend B and C can cross (in 5 minutes), and Friend B can go back across the bridge to give the flashlight to Friends A and D (in another 2 minutes).
**Given the 2 person maximum on the bridge and the requirement to travel with a flashlight, what is the fastest time in which all 4 friends can safely cross the bridge to the mainland?**
*Hint: the solution doesn't involve throwing the flashlight, hanging off the bridge, shining the flashlight from the mainland to the island, etc.*2013-02-09T17:54:02Ztag:www.mindcipher.com,2005:Puzzle/1292013-02-06T03:04:12Z2013-04-22T21:54:06Z/puzzles/129Bit Pair SwapIn one line of C or Java, how would you swap every pair of bits in a byte? That is, swap bits 0 and 1, 2 and 3, 4 and 5, 6 and 7.
Example: `0110 1010` becomes `1001 0101`.2013-02-06T03:04:12Ztag:www.mindcipher.com,2005:Puzzle/1282013-02-04T16:10:28Z2014-03-09T05:36:00Z/puzzles/128Products AlgorithmGiven an array *A[1...N]* of integers, create an array *O[1...N]* where *O[i]* = the product of all elements in A except for the ith element.
For example, if *N* = 3, and *A* = {6, 4, 2}, *O* = {8, 12, 24}
The tricky part: don't use divide and do it in *O(N)*.
The solution might be well known, but I thought it was interesting.2013-02-04T16:10:28Ztag:www.mindcipher.com,2005:Puzzle/1272013-02-02T00:29:09Z2013-12-17T12:41:57Z/puzzles/127The double-square number problemIt's said that a number N with 4 digits is a double-square number when it equals the sum of the squares of two numbers: one formed by the first two digits of N, in the order they appear in N and the other formed by the two last digits of N in the order they appear in N.
For example, 1233 is a double-square number since 1233 = 12^2 + 33^2. Find another double-square number.2013-02-02T00:29:09Ztag:www.mindcipher.com,2005:Puzzle/1262013-02-01T20:07:36Z2013-05-02T12:54:43Z/puzzles/1263 Coins*Not* sure what the answer is, ideas welcome!
You have 3 coins, one always comes up heads, the second always comes up tails, and the third is a fair coin. You select a coin at random. After selecting a coin, you flip it twice and get heads both times. What is the probability that your next flip is heads?
**Follow up:**
Suppose you flipped twice and got the *same* outcome both times (either HH or TT), then what's the probability that your next two flips match the first two (e.g. if you saw HH, your next two are HH)2013-02-01T20:07:36Ztag:www.mindcipher.com,2005:Puzzle/1252013-01-29T21:26:56Z2014-08-22T00:11:28Z/puzzles/125Unfair CoinSay you have an unfair coin: a coin whose probability of flipping heads and flipping tails is unknown but nonzero.
Can you design a game where you and your opponent have an equal chance of winning?2013-01-29T21:26:56Ztag:www.mindcipher.com,2005:Puzzle/1242013-01-27T06:01:14Z2013-11-26T01:41:33Z/puzzles/124Carpeting a DonutGiven two concentric circles and a chord of the outer circle, tangent to the inner circle, of length 25m, find the area between the two circles. See illustration. No other information needed, all you need is the length of that chord.2013-01-27T06:01:14Ztag:www.mindcipher.com,2005:Puzzle/1232013-01-26T06:14:45Z2014-08-19T06:20:17Z/puzzles/123Cocktail PartyYou are observing a cocktail party that has seven guests. You notice that each guest shakes hands with one another once they meet, but never again. How many handshakes occurred during the cocktail party?2013-01-26T06:14:45Ztag:www.mindcipher.com,2005:Puzzle/1222013-01-26T06:03:47Z2014-06-03T23:39:06Z/puzzles/122Going to the TrackThe London racetrack needs to submit its top three horses to the Kentucky Derby next month in order to compete for a prize. However in a recent electrical storm, all the racetrack's previous race history was erased such that no one knows the previous times of any of the horses. To make matters worse, each horse looks identical and it is impossible to remember which ones were the fastest.
London racetrack is home to 25 horses, but their track can only race 5 horses at a time. What is the fewest number of races that can be conducted in order to determine the 3 fastest horses?2013-01-26T06:03:47Ztag:www.mindcipher.com,2005:Puzzle/1212013-01-25T04:11:44Z2014-08-19T06:14:43Z/puzzles/121All the King's WineSo there's this king. Someone breaks into his wine cellar where he stores 1000 bottles of wine. This person proceeds to poison one of the 1000 bottles, but gets away too quickly for the king's guard to see which one he poisoned or to catch him.
The king needs the remaining 999 safe bottles for his party in 4 weeks. The king has 10 servants who he considers disposable. The poison takes about 3 weeks to take effect, and any amount of it will kill whoever drinks it. How can he figure out which bottle was poisoned in time for the party?2013-01-25T04:11:44Ztag:www.mindcipher.com,2005:Puzzle/1202013-01-23T09:44:24Z2013-09-01T12:59:21Z/puzzles/120100 Light BulbsThere are 100 light switches, all of them are off. First, you walk by them, turning all of them on. Next, you walk by them turning every other one off. Then, you walk by them changing every third one. On your 4th pass, you change every 4th one.
You repeat this for 100 passes. At the end, how many lights will be on?2013-01-23T09:44:24Ztag:www.mindcipher.com,2005:Puzzle/1032013-01-09T01:39:55Z2013-11-18T20:59:21Z/puzzles/103Balloon in an Elevator
<p>A person is holding on to a helium balloon in an elevator. The elevator cable snaps, and at the exact same moment, the person lets go of the helium balloon. In the perspective of the person in the elevator, what happens to the balloon the moment the cable snaps?</p>
2013-01-09T01:39:55Ztag:www.mindcipher.com,2005:Puzzle/1022013-01-09T01:39:32Z2013-08-20T00:59:37Z/puzzles/102Betsy and Patsy
<p>“We were born on the same day of the same year,” said Betsy.
“And we have the same mother and father,” said Patsy.
“But we’re not twins!” the two exclaimed.
Can you explain?</p>
2013-01-09T01:39:32Ztag:www.mindcipher.com,2005:Puzzle/1012013-01-09T01:38:00Z2014-08-16T09:58:31Z/puzzles/101Prisoners and hats
<p>100 prisoners are standing in a line each 1 seeing the back of the ones in front of him. Each prisoner is given a black or a white hat. If they can guess the color of their hat they can go away freely, but if they say anything else than black or white or do anything else then everyone stays in prison. From the last one to the first each prisoner is asked 1 by 1 their hat color. The question is what tactic should they use so that the most people get out with certainty and whats the maximum number of people that can get out of prison(this number can be checked).</p>
2013-01-09T01:38:00Ztag:www.mindcipher.com,2005:Puzzle/992013-01-09T01:34:32Z2013-08-20T01:07:42Z/puzzles/99The Female StarMy first two letters are a man. The first three a woman. The first four are a brave man. And in whole I am a brave woman.
2013-01-09T01:34:32Ztag:www.mindcipher.com,2005:Puzzle/982013-01-09T01:32:43Z2013-12-06T07:44:36Z/puzzles/98All Apples
<p>On the market, mrs. Jones and mrs. Smith sell apples. Mrs. Jones sells her apples per two for 0.50 euro. The apples of Mrs. Smith are a bit smaller; she sells hers per three for 0.50 euro. At a certain moment, when both ladies have the same amount of apples left, Mrs. Smith is being called away. She asks her neighbour to take care of her goods. To make everything not too complicated, Mrs. Jones puts all apples to one big pile, and starts selling them for one euro per five apples. When Mrs. Smith returns at the end of the day, all apples have been sold. But when they start dividing the money, there appears to be a shortage of 3.50 euro.</p>
<p>Supposing they divide the amount of money equally, how much does mrs. Jones lose with this deal?</p>
2013-01-09T01:32:43Ztag:www.mindcipher.com,2005:Puzzle/972013-01-09T01:31:01Z2013-05-29T14:04:12Z/puzzles/97Interview Question - DatabasesThis is a common interview question at companies like Google and Amazon:
<p>“Explain a database in three sentences to your eight-year-old nephew.”</p>
<p>This question has an infinite number of solutions. Feel free to comment below and add your own answer!</p>
<p>This was an actual question asked at an interview with Google.</p>
2013-01-09T01:31:01Ztag:www.mindcipher.com,2005:Puzzle/962013-01-09T01:28:15Z2014-06-21T09:26:45Z/puzzles/96The Cabin
<p>There is a man lying face-down, dead, in the middle of a cabin. The cabin is in the middle of the woods. There are windows, but they do not open. What is going on?</p>
2013-01-09T01:28:15Ztag:www.mindcipher.com,2005:Puzzle/952013-01-09T01:26:33Z2014-01-14T07:23:50Z/puzzles/95The Tallest Mountain
<p>A man approaches you with a very enigmatic look on his face. He asks your for a bit of your time and if you could answer a quick question for him. He asks: “Before Mt. Everest was discovered, which was the tallest mountain in the world?”</p>
<p>So you answered:…</p>
2013-01-09T01:26:33Ztag:www.mindcipher.com,2005:Puzzle/932013-01-09T01:23:02Z2013-08-20T01:07:11Z/puzzles/93Gone Camping<p>Camp Crona’s cook, Allison, was just about to begin preparing the picnic lunch for all the campers. She already knew she needed to fill 55 bowls of the same size and capacity with the same amount of food. When she was done, she decided to read the guidelines for the picnic, just out of curiosity. The guidelines said:</p>
<p>1. Every camper gets their own bowl of soup.
2. Every two campers will get one bowl of spaghetti to share.
3. Every three campers will get one bowl of salad to share.
4. All campers are required to have their own helping of salad, spaghetti, and soup.</p>
<p>After some rapid calculations, Margaret was able to figure out how many campers were going to the picnic. Can you?</p>
2013-01-09T01:23:02Ztag:www.mindcipher.com,2005:Puzzle/922013-01-09T01:22:55Z2013-11-13T20:00:29Z/puzzles/92Are the lines parallel?
<p>Are the horizontal lines in the picture parallel?</p>
2013-01-09T01:22:55Ztag:www.mindcipher.com,2005:Puzzle/912013-01-09T01:19:55Z2013-05-29T14:06:56Z/puzzles/91Find the Missing Number
<p>In this table, each row across follows the same pattern of numbers. See if you can discern the pattern and fill in the missing number in the bottom row.</p>
<p>7 4 8</p>
<p>3 9 7</p>
<p>6 5 10</p>
<p>? 8 4</p>2013-01-09T01:19:55Ztag:www.mindcipher.com,2005:Puzzle/902013-01-09T01:18:32Z2013-10-08T12:36:56Z/puzzles/90The Middle of Nowhere
<p>What is in the middle of "nowhere"?</p>
<p>The answer is indisputable, and not based on some philosophy.</p>
2013-01-09T01:18:32Ztag:www.mindcipher.com,2005:Puzzle/892013-01-09T01:17:50Z2013-12-10T11:16:59Z/puzzles/89Attic Light Switch
<p>A downstairs panel contains three on-off switches. One of them controls the lamp in your attic. Using only ONE trip up to the attic, how do you determine which switch controls the lamp?</p>
2013-01-09T01:17:50Ztag:www.mindcipher.com,2005:Puzzle/882013-01-09T01:15:53Z2014-03-04T21:35:40Z/puzzles/88Clock Hands - Google Interview Question
<p>How many degrees are there in the angle between the hour and minute hands of a clock when the time is a quarter past three?</p>
2013-01-09T01:15:53Ztag:www.mindcipher.com,2005:Puzzle/862013-01-09T01:12:34Z2013-08-20T01:28:52Z/puzzles/86Salary
<p>Three coworkers would like to know their average salary. how can they do it, without disclosing their own salaries.</p>
2013-01-09T01:12:34Ztag:www.mindcipher.com,2005:Puzzle/852013-01-09T01:10:25Z2013-09-30T15:50:12Z/puzzles/85Tea and Wine
<p>You take a spoon of wine from a barrel of wine and put it into your cup of tea. Then you return a spoon of the (non-uniform) mixture of tea and wine from your cup and put it in the barrel. Now you have some foreign substance (wine) in the cup and some foreign substance (tea) in the barrel. Which is larger: the quantity of wine in the cup or the quantity of tea in the barrel?</p>
2013-01-09T01:10:25Ztag:www.mindcipher.com,2005:Puzzle/842013-01-09T01:08:47Z2013-04-11T21:56:31Z/puzzles/84Balloon Business
<p>You’re sitting in a stationary car. There’s a helium balloon tied to its floor. You accelerate and, obviously feel like you’re being pushed backwards (against the direction of your acceleration). Which way will the balloon move?</p>
<p>Note: I found this puzzle and its explanation at physicallyincorrect.com</p>
2013-01-09T01:08:47Ztag:www.mindcipher.com,2005:Puzzle/832013-01-09T01:06:26Z2014-08-15T12:05:59Z/puzzles/83Nice to meet you
<p>At a secret meeting, everyone shakes hands exactly once with every other person present.
Altogether there are forty-five handshakes. How many people attended the meeting?</p>
2013-01-09T01:06:26Ztag:www.mindcipher.com,2005:Puzzle/812013-01-09T01:03:10Z2014-01-27T22:04:58Z/puzzles/81Time To Fuse
<p>You have a string-like fuse that burns in exactly one minute. The fuse is inhomogeneous, and it may burn slowly at first, then quickly, then slowly, and so on. You have a match, and no watch. How do you measure exactly 30 seconds?</p>
2013-01-09T01:03:10Ztag:www.mindcipher.com,2005:Puzzle/802013-01-09T01:02:00Z2013-08-20T05:26:16Z/puzzles/80Boys and Girls
<p>To help solve an acute national over-population problem, a government issues a radical decree that families must stop having children as soon as they have had one son. After a while, there are some families with only one boy, some with one girl and one boy, some with two girls and one boy, some with three girls and one boy, some with six girls and one boy, etc., and some families only have girls and give up trying for a son.</p>
<p>Would boys tend to outnumber girls in this generation? Or vice versa?</p>
2013-01-09T01:02:00Ztag:www.mindcipher.com,2005:Puzzle/792013-01-09T01:00:46Z2013-12-31T10:08:53Z/puzzles/79Bug's Eye
<p>You are a bug sitting in one corner of a cubic room. You wish to walk (not fly) to the extreme opposite corner – the corner furthest from you. Describe the shortest path that you can walk.</p>
2013-01-09T01:00:46Ztag:www.mindcipher.com,2005:Puzzle/782013-01-09T00:59:31Z2014-01-27T23:26:31Z/puzzles/78Measuring Up
<p>You have a 3 and a 5 litre water container, each container has no markings except for that which gives you its total volume. You also have a running tap. You must use the containers and the tap in such away as to exactly measure out 4 litres of water. How is this done?</p>
2013-01-09T00:59:31Ztag:www.mindcipher.com,2005:Puzzle/772013-01-09T00:56:40Z2013-09-02T19:45:28Z/puzzles/77Don't Hang Yourself
<p>A fool wants to tie a rope around the earth. So he buys a rope of 40,000 KM and ties it around the world. His neighbour, also a fool, wants to do the same only he wants the rope on sticks 1 meter above the ground.</p>
<p>How much more rope does he need?
And how much more rope do you need when you use a tennis ball instead of the earth</p>
2013-01-09T00:56:40Ztag:www.mindcipher.com,2005:Puzzle/762013-01-09T00:55:33Z2013-03-22T03:56:00Z/puzzles/76Random Puzzle
<p>Here’s something I heard from a friend, I thought it was appropriate.</p>
<p>What do you call a 4 foot tall fortune teller who is wanted by the law?</p>
<p>Good luck.</p>
2013-01-09T00:55:33Ztag:www.mindcipher.com,2005:Puzzle/752013-01-09T00:54:26Z2013-05-12T16:43:37Z/puzzles/75Parodoxical Statements from Life
<p>Nobody goes to that restaurant; it’s too crowded.</p>
<p>Don’t go near the water ’til you have learned how to swim.</p>
<p>The man who wrote such a stupid sentence can not write at all.</p>
<p>If you get this message, call me, and if you don’t get it, don’t call.</p>
<p>and as a bonus:
Answer truthfully (yes or no) to the following question: Will the next word you say be ‘no’?</p>
2013-01-09T00:54:26Ztag:www.mindcipher.com,2005:Puzzle/742013-01-09T00:51:28Z2013-08-20T05:36:03Z/puzzles/74Ball Drop
<p>A man is floating in a hot air balloon off the coast of Ireland.</p>
<p>In one hand he has a bowling ball, and in the other a golf ball, and he drops them both over the side at the same time.</p>
<p>which one will hit the ground first?</p>
2013-01-09T00:51:28Ztag:www.mindcipher.com,2005:Puzzle/732013-01-09T00:51:03Z2013-11-16T00:26:16Z/puzzles/73Marble Weights
<p>You are given a set of scales and 12 marbles. The scales are of the old balance variety. That is, a small dish hangs from each end of a rod that is balanced in the middle. The device enables you to conclude either that the contents of the dishes weigh the same or that the dish that falls lower has heavier contents than the other.
The 12 marbles appear to be identical. In fact, 11 of them are identical, and one is of a different weight. Your task is to identify the unusual marble and discard it. You are allowed to use the scales three times if you wish, but no more.</p>
<p>Note that the unusual marble may be heavier or lighter than the others. You are asked to both identify it and determine whether it is heavy or light.</p>
2013-01-09T00:51:03Ztag:www.mindcipher.com,2005:Puzzle/722013-01-09T00:49:18Z2013-12-02T19:42:51Z/puzzles/72Troll Toll
<p>You are on your way to visit your Grandma, who lives at the end of the valley. It’s her birthday, and you want to give her the cakes you’ve made.</p>
<p>Between your house and her house, you have to cross 7 bridges, and as it goes in the land of make believe, there is a troll under every bridge! Each troll, quite rightly, insists that you pay a troll toll. Before you can cross their bridge, you have to give them half of the cakes you are carrying, but as they are kind trolls, they each give you back a single cake.</p>
<p>How many cakes do you have to leave home with to make sure that you arrive at Grandma’s with exactly 2 cakes?</p>
2013-01-09T00:49:18Ztag:www.mindcipher.com,2005:Puzzle/712013-01-09T00:46:27Z2014-02-22T20:36:49Z/puzzles/71What is Sammy exactly?
<p>Police! Police!
She cried,
Police! he died!
Sammy was my friend.</p>
<p>If this is the end
and i wont see him again,
let me give just a kiss.
Sammy was my friend.</p>
<p>Down the roar he shall slide
turning round and round
from side to side
Sammy was my friend.</p>
<p>Whenever I’m on the potty
Ill think of his little gold body
taking its very last swim
Sammy was my friend</p>
<p>Through all the good times
and the troubles,
i will never forget Sammy’s bubbles.
After all, Sammy was my friend.</p>
2013-01-09T00:46:27Ztag:www.mindcipher.com,2005:Puzzle/702013-01-09T00:46:05Z2013-05-29T16:13:23Z/puzzles/70Sold!
<p>A dealer bought a painting for $70, sold it for $80, bought it back for $90, and sold it again for $100.
How much profit did he make?</p>
2013-01-09T00:46:05Ztag:www.mindcipher.com,2005:Puzzle/692013-01-09T00:43:15Z2013-05-01T09:25:41Z/puzzles/69Cracking the Safe
<p>You are to open a safe without knowing the combination. Beginning with the dial set at zero, the dial must be turned counter-clockwise to the first combination number, (then clockwise back to zero), and clockwise to the second combination number, (then counter-clockwise back to zero), and counter-clockwise again to the third and final number, where upon the door shall immediately spring open. There are 40 numbers on the dial, including the zero.</p>
<p>Without knowing the combination numbers, what is the maximum number of trials required to open the safe (one trial equals one attempt to dial a full three-number combination)?</p>
2013-01-09T00:43:15Ztag:www.mindcipher.com,2005:Puzzle/682013-01-09T00:42:08Z2013-05-29T15:56:21Z/puzzles/68The Twiddled Bolts
<p>Two identical bolts are placed so that their helical grooves intermesh. If you move the bolts around each other as shown in the picture, would they
a) move inward
b) move outward
c) remain the same distance from each other</p>
2013-01-09T00:42:08Ztag:www.mindcipher.com,2005:Puzzle/672013-01-09T00:41:13Z2013-08-20T01:02:45Z/puzzles/67The Classic Weights & Scale
<p>You have 8 lead marbles, one of which is heavier than the rest. What is the minimum amount of “weightings” (i.e. with a see-saw scale) you need to do to guarantee finding the odd one out?</p>
2013-01-09T00:41:13Ztag:www.mindcipher.com,2005:Puzzle/662013-01-09T00:39:05Z2013-02-02T01:08:40Z/puzzles/663 Painted Ladies
<p>There are 3 smart old ladies standing in a desert such a way that each lady can see the other two. Each of the 3 ladies has paint on her face. Seeing this, all three ladies begin laughing. After a while, the smartest of them realizes something, and stops laughing. Why? And more importantly, HOW did she realize it?</p>
<p>No, there are no mirrors, they can’t feel the paint on their own face, they don’t speak to each other, there is nothing in the desert, etc. There is nothing physical about this puzzle, it is just logical.</p>
2013-01-09T00:39:05Ztag:www.mindcipher.com,2005:Puzzle/652013-01-09T00:37:01Z2013-09-15T21:29:24Z/puzzles/65Farmer's Market
<p>You’re a farmer. You’re going to a market to buy some animals. On the market there are 3 types of animals for sale. You can buy:</p>
<p>Horses for £10 each, goats for £1 each and ducks, you get 8 of these per bunch and each bunch costs £1.</p>
<p>The aim is to acquire 100 animals at the cost of £100, what is the combination of horses, goats and duck that allows you to do this? (you must buy at least one of each.)</p>
2013-01-09T00:37:01Ztag:www.mindcipher.com,2005:Puzzle/642013-01-09T00:35:28Z2014-02-23T03:35:30Z/puzzles/64The fisherman and the Hunter
<p>The are two men that always tell the truth except for the days when they are lying. The fisherman lies on Monday, Tuesday and Wednesday. The Hunter lies on Thursday, Friday and Saturday. Today they say, they were both lying yesterday. What day is today?</p>
2013-01-09T00:35:28Ztag:www.mindcipher.com,2005:Puzzle/632013-01-09T00:33:27Z2013-02-02T01:08:39Z/puzzles/63My eyes
<p>My eyes fold like telescopes
and view the world like periscopes
Who am I?</p>
<p>For a hint you should know
I run really slow
and walk as fast as i can.
Do you know who I am?</p>
2013-01-09T00:33:27Ztag:www.mindcipher.com,2005:Puzzle/622013-01-09T00:32:04Z2013-09-02T20:09:18Z/puzzles/62Running Race
<p>Phil and Sal race each other for 100 yards. Sal wins by 10 yards. They decide
to race again, but this time, to even things up, Sal begins 10 yards behind the start
line. Assuming that both run with the same constant speed as before, who wins?</p>
2013-01-09T00:32:04Ztag:www.mindcipher.com,2005:Puzzle/602013-01-09T00:28:47Z2014-02-24T18:09:13Z/puzzles/60Fork in the road
<p>You are walking down a road and arrive at a fork that splits the road into two separate paths. When you reach, you meet two identical twins, each standing at the edge of one of the two paths. You have heard that there is a bag with $1,000,000 at the end of one path and you are told that one of the two twins always tells the truth and the other always lies. There is no way you can get to the end of both roads before the sun goes down.</p>
<p>What ONE question can you ask (of only one twin) that would lead you to the money?</p>
2013-01-09T00:28:47Ztag:www.mindcipher.com,2005:Puzzle/592013-01-09T00:27:37Z2014-03-06T21:20:28Z/puzzles/59Birthday Problem (Classic)
<p>A classic problem.</p>
<p>How many people do you need to have in a room so that the probability of at least 2 of people sharing a birthday is greater than 50%.</p>
<p>Assume 365 days in a year (no leap years), and birthdays are random over these 365 days, and people born in different years on the same day obviously have the same birthday.</p>
<p>Also, no twins, twins are cheating.</p>
2013-01-09T00:27:37Ztag:www.mindcipher.com,2005:Puzzle/582013-01-09T00:24:47Z2013-11-26T02:05:29Z/puzzles/58Coin Flip
<p>On Monday, you flip a coin all day. You start flipping it until you see the pattern Head, Tail, Head. You record the number of flips required to reach this pattern, and start flipping again (and counting up from 1 again) until you see that pattern again, you record the second number, and start again. At the end of the day you average all of the numbers you’ve recorded.
On Tuesday you do the EXACT same thing except you flip until you see the pattern Head, Tail, Tail.</p>
<p>Will Monday’s number be higher than Tuesday’s, equal to Tuesday’s, or lower than Tuesday’s?</p>
<p></p>
2013-01-09T00:24:47Ztag:www.mindcipher.com,2005:Puzzle/572013-01-09T00:24:33Z2014-03-06T21:17:29Z/puzzles/57Microsoft Interview Question - Count it Out
<p>This problem is more targeted towards people with Computer Science or similar backgrounds, it is a REAL question from an interview with Microsoft.</p>
<p>You have an array of 99 unique numbers from 0-100. What is the fastest (in computing) way to find the number between 0 and 100 that is not in the array?</p>
2013-01-09T00:24:33Ztag:www.mindcipher.com,2005:Puzzle/562013-01-09T00:22:05Z2014-08-19T06:25:38Z/puzzles/56Happy Handshaking
<p>Jack and his wife went to a party where four other married couples were present. Every person shook hands with everyone he or she was not acquainted with. When the handshaking was over, Jack asked everyone, including his own wife, how many hands they shook. To his surprise, Jack got nine different answers.</p>
<p>How many hands did Jack’s wife shake?</p>
2013-01-09T00:22:05Ztag:www.mindcipher.com,2005:Puzzle/552013-01-09T00:20:17Z2014-08-15T12:04:38Z/puzzles/55Electrical Deaths
<p>You and your friend are driving out in the pouring rain one night. It soon becomes much to hard too see, so you stop at closest house possible. When you knock on the door, an old lady welcomes you to stay for the night. You notice that the power is out. You go to sleep on the lady’s comfortable bed.</p>
<p>The rain wakes you up in the middle of the night, and you find yourself chained to the wall. The lady is sitting there, saying “choose wisely, or you will die…”</p>
Do you choose… the red, green, or blue door?
Now, do you choose the yellow, orange, or purple hall?
Sadly, that doesn’t matter and you die. You have the choice of a death by electric eels or electric chair. Which do you choose?
2013-01-09T00:20:17Ztag:www.mindcipher.com,2005:Puzzle/542013-01-09T00:18:22Z2014-01-02T02:27:40Z/puzzles/54Tick Tock
<p>With a seven-minute hourglass and an eleven-minute hourglass,
what is the quickest way to time the boiling of an egg for fifteen minutes?</p>
2013-01-09T00:18:22Ztag:www.mindcipher.com,2005:Puzzle/532013-01-09T00:17:32Z2013-01-26T23:24:31Z/puzzles/53Don't be a Square
<p>Shown is a large square made up of four shapes, and a separate square, labeled with an E. Using all five of the shapes (A through E) only once, can you form a second perfect square?</p>
2013-01-09T00:17:32Ztag:www.mindcipher.com,2005:Puzzle/522013-01-09T00:16:12Z2013-01-28T21:30:27Z/puzzles/52What Am I?
<p>Give me food, and I will live
Give me water, and I will die</p>
<p>What am I?</p>
2013-01-09T00:16:12Ztag:www.mindcipher.com,2005:Puzzle/502013-01-09T00:11:32Z2013-08-20T04:52:49Z/puzzles/50Its waistline is a hexagon
<p>Heart of Darkness,
Coat of Grain,
Was once alive,
Now feels no pain,
Never walks, but
Leaves a Trail.
Circle head and pointed tail.
Grows shorter as the day goes on-
Its waistline is a hexagon.</p>
<p>What am I?</p>
2013-01-09T00:11:32Ztag:www.mindcipher.com,2005:Puzzle/492013-01-09T00:10:02Z2014-05-19T00:27:22Z/puzzles/49Don't Mate in One
<p>This problem was first publicized by Karl Fabel, a German chess problemist.</p>
<p>Find a move for white that will not result in an immediate checkmate of the black king.</p>
<p></p>
2013-01-09T00:10:02Ztag:www.mindcipher.com,2005:Puzzle/482013-01-09T00:09:37Z2014-05-19T00:44:29Z/puzzles/48Divide 1 to 20
<p>What is the smallest whole number that can be divided by any number from 1 to 20 (inclusive) and not leave a remainder? For example, 60 is the smallest number that can be divided evenly by all the numbers from 1 – 5.</p>
<p>Note: This problem can be solved in only a couple of minutes using only pen and paper.</p>
2013-01-09T00:09:37Ztag:www.mindcipher.com,2005:Puzzle/472013-01-09T00:07:30Z2014-08-16T13:45:26Z/puzzles/47The Neighbourhood
<p>People say Einstein suggested that 98% of the world’s population couldn’t figure out the answer to this puzzle. It is not for the faint of heart:</p>
<p>There are 5 houses each with a different color. Their owners, each with a unique heritage, drinks a certain type of beverage, smokes a certain brand of cigarette, and keeps a certain variety of pet. None of the owners have the same variety of pet, smoke the same brand of cigarette or drink the same beverage.</p>
<p>Clues:</p>
* The Brit lives in the red house.
* The Swede keeps dogs as pets.
* The Dane drinks tea.
* Looking from in front, the green house is just to the left of the white house.
* The green house’s owner drinks coffee.
* The person who smokes Pall Malls raises birds.
* The owner of the yellow house smokes Dunhill.
* The man living in the center house drinks milk.
* The Norwegian lives in the leftmost house.
* The man who smokes Blends lives next to the one who keeps cats.
* The man who keeps a horse lives next to the man who smokes Dunhill.
* The owner who smokes Bluemasters also drinks beer.
* The German smokes Prince.
* The Norwegian lives next to the blue house.
* The man who smokes Blends has a neighbor who drinks water.
<p>(Note: I found this puzzle and its solution at
http://www.brain-fun.com/Brain-Teasers/EinsteinsRiddle.php)</p>
2013-01-09T00:07:30Ztag:www.mindcipher.com,2005:Puzzle/462013-01-09T00:05:35Z2014-03-06T21:15:21Z/puzzles/46Good luck at an interview with Google
I just came across these Google interview questions… impossible:
* How many golf balls can fit in a school bus?
* How much should you charge to wash all the windows in Seattle?
* How many piano tuners are there in the entire world?
How are you supposed to answer these!?
2013-01-09T00:05:35Ztag:www.mindcipher.com,2005:Puzzle/442013-01-09T00:01:49Z2013-08-20T01:02:08Z/puzzles/44Guilty Innocence
<p>I am a drug dealer. I have been questioned by the police with all of my drugs surrounding me, but I was not arrested. Why?</p>
2013-01-09T00:01:49Ztag:www.mindcipher.com,2005:Puzzle/432013-01-09T00:00:10Z2014-02-08T16:00:34Z/puzzles/43Stuck in a brick building
<p>A man is stuck inside an all brick building (floor, ceiling, and walls are all brick), that has no windows and no doors, and inside with him he has only a wooden board and a saw, how does he get out?</p>
2013-01-09T00:00:10Ztag:www.mindcipher.com,2005:Puzzle/422013-01-08T23:58:29Z2014-01-13T16:22:47Z/puzzles/42The Stealing Mailman
<p>This is a fairly simple problem that can cause some people trouble.
Kevin and Sophia want to mail high confidential letters between each other. They know that the mailman is crooked and opens the letters. Each of them has their own key and matching lock, and Sophia has a wooden box she plans on locking.</p>
<p>Neither key can be used on the other lock. Using the box, how can the two successfully mail letters between each other without the mailman seeing?</p>
2013-01-08T23:58:29Ztag:www.mindcipher.com,2005:Puzzle/412013-01-08T23:56:27Z2014-03-28T10:34:17Z/puzzles/41Clever Commuter
<p>Every day, Fred takes the train to travel from his work back to Alkmaar, his place of residence. Usually, he arrives at the station of Alkmaar at six o’clock, and exactly at that moment he is picked up by his wife by car. Yesterday evening, Fred took an earlier train, without informing his wife, and therefore he already was at the station of Alkmaar at five o’clock. He decided to walk part of the way to meet his wife. When he met the car with his wife, he drove home with her. In this way, they were home ten minutes earlier than normal.</p>
<p>How long did Fred walk yesterday evening?</p>
2013-01-08T23:56:27Ztag:www.mindcipher.com,2005:Puzzle/402013-01-08T23:55:50Z2013-05-01T09:23:34Z/puzzles/40Left Hands Only
<p>Assume there are approximately 6,000,000,000 (6 billion) people on Earth. What would you estimate to be the result if you multiply together the number of fingers on every person’s left-hands?</p>
<p>For the purposes of this exercise, thumbs count as fingers, for five fingers per hand. If you cannot estimate the number then try to guess how long the number would be.</p>
2013-01-08T23:55:50Ztag:www.mindcipher.com,2005:Puzzle/392013-01-08T23:53:40Z2013-08-20T02:18:51Z/puzzles/39The Easy Sum of 1..100?
<p>Quick,without using a calculator, what is the sum of all numbers between 1 and 100 inclusive?</p>
<p>Or 1..10 for that matter..</p>
2013-01-08T23:53:40Ztag:www.mindcipher.com,2005:Puzzle/382013-01-08T23:52:41Z2013-09-15T21:52:08Z/puzzles/38What number is next?
<p>What number comes next in this sequence?</p>
<p>4, 20, 56, 120, 220, ?</p>
2013-01-08T23:52:41Ztag:www.mindcipher.com,2005:Puzzle/372013-01-08T23:51:18Z2013-10-01T13:20:23Z/puzzles/37Power of 2 (Microsoft Interview)Given any number n, using one line of C (using no external libraries or functions) code, determine if n is a power of 2. Return 1 if n is a power of 2 and 0 if not.
2013-01-08T23:51:18Ztag:www.mindcipher.com,2005:Puzzle/362013-01-08T23:48:14Z2014-08-16T08:21:35Z/puzzles/36A Pharmacist's Nightmare
<p>You have 5 jars of pills. Each pill weighs 10 gram, except for contaminated pills contained in one jar, where each pill weighs 9 gm. Given a scale, how could you tell which jar had the contaminated pills in just one measurement?</p>
2013-01-08T23:48:14Ztag:www.mindcipher.com,2005:Puzzle/352013-01-08T23:47:33Z2013-08-20T04:22:59Z/puzzles/35Cutting Numbers
<p>When is Seven half of Twelve?</p>
2013-01-08T23:47:33Ztag:www.mindcipher.com,2005:Puzzle/342013-01-08T23:46:00Z2013-08-20T04:39:20Z/puzzles/34Plane In The Wind
<p>An airplane flies in a straight line from airport A to airport B and back. It has constant engine speed and no wind.</p>
<p>Will its total flight time increase, decrease or stay the same if throughout both flights there is a constant wind blowing from A to B?</p>
2013-01-08T23:46:00Ztag:www.mindcipher.com,2005:Puzzle/332013-01-08T23:44:10Z2013-04-12T03:08:13Z/puzzles/33Lockers
<p>A high school has a strange principal. On the first day, he has his students perform an odd opening day ceremony:</p>
<p>There are one thousand lockers and one thousand students in the school. The principal asks the first student to go to every locker and open it. Then he has the second student go to every second locker and close it. The third goes to every third locker and, if it is closed, he opens it, and if it is open, he closes it. The fourth student does this to every fourth locker, and so on. After the process is completed with the thousandth student, how many lockers are open?</p>
2013-01-08T23:44:10Ztag:www.mindcipher.com,2005:Puzzle/312013-01-08T23:40:01Z2014-04-01T16:48:32Z/puzzles/31The Way Engineers Think
<p>A priest, a doctor, and an engineer were waiting one morning for a particularly slow group of golfers.</p>
<p>Engineer: What’s with these guys? We must have been waiting for 15 minutes!</p>
<p>Doctor: I don’t know but I’ve never seen such ineptitude!</p>
<p>Priest: Hey, here comes the greenskeeper. Let’s have a word with him.</p>
<p>Priest: Hi George. Say George, what’s with that group ahead of us? They’re rather slow aren’t they?</p>
<p>George: Oh yes. That’s a group of blind fire fighters. They lost their sight while saving our club house last year. So we let them play here anytime free of charge!</p>
<p>(silence)</p>
<p>Priest: That’s so sad. I think I will say a special prayer for them tonight.</p>
<p>Doctor: Good idea. And I’m going to contact my ophthamologist buddy and see if there’s anything he can do for them.</p>
<p>What does the engineer say?</p>
2013-01-08T23:40:01Ztag:www.mindcipher.com,2005:Puzzle/292013-01-08T23:37:56Z2014-08-17T21:18:14Z/puzzles/29Up and Down
<p>Explain the following scenario:</p>
<p>A man lives on the twelfth floor of an apartment building. Every morning he takes the elevator down to the lobby and leaves the building. In the evening, he gets into the elevator, and, if there is someone else in the elevator — or if it was raining that day — he goes back to his floor directly. Otherwise, he goes to the tenth floor and walks up two flights of stairs to his apartment.</p>
2013-01-08T23:37:56Ztag:www.mindcipher.com,2005:Puzzle/282013-01-08T23:35:49Z2014-02-19T19:14:19Z/puzzles/2812 Identical Balls
<p>You have 12 identical balls. One of them is slightly different in weight than the rest. You have three weighs of a balance scale to determine which ball is different and whether it is heavier or lighter.</p>
<p>This is not easy… try hard to figure this out before looking at the answer.</p>
2013-01-08T23:35:49Ztag:www.mindcipher.com,2005:Puzzle/272013-01-08T23:34:38Z2013-08-20T06:16:21Z/puzzles/27Classic Knights & Knaves
<p>Raymond Smullyan has made a career of writing logic problems. His most famous have to do with a mythical island of Knights and Knaves. Knights always tell the truth, and Knaves always lie.</p>
<p>As you approach the island, you spot three inhabitants on the shore. You call out to them, “Are you Knights or Knaves?” The first says something but you do not hear what he says, so you ask, “What did you say?” The second inhabitant says, “He says he is a Knight, he is and so am I.” The third responds, “He is a Knave, but I am a Knight.” Figure out the true identities of the three individuals.</p>
2013-01-08T23:34:38Ztag:www.mindcipher.com,2005:Puzzle/262013-01-08T23:31:33Z2013-07-23T13:13:52Z/puzzles/26Cooking Time
<p>A cook must put a souffle exactly 9 minutes at the oven, but he only have a 7 minutes hourglass and a 4 minutes hourglass.</p>
<p>How can he measure 9 minutes with those clocks?</p>
2013-01-08T23:31:33Ztag:www.mindcipher.com,2005:Puzzle/252013-01-08T23:31:21Z2013-05-05T01:07:45Z/puzzles/25Right or Left?This is interesting. Look at the animation below.
Do you see the girl spinning clockwise or counter-clockwise
2013-01-08T23:31:21Ztag:www.mindcipher.com,2005:Puzzle/242013-01-08T23:28:05Z2013-11-18T20:37:16Z/puzzles/24The Price Is Right
<p>Many shops have prices set just under a round figure, e.g. $9.99 instead of $10.00 or $99.95 instead of $100.00 . It is assumed that this is done because the price seems lower to the consumer. But this is not the reason the practice started. What was the original reason for this pricing method?</p>
2013-01-08T23:28:05Ztag:www.mindcipher.com,2005:Puzzle/232013-01-08T23:27:59Z2013-11-26T19:38:33Z/puzzles/23Simple 24
<p>Why is that if p is a prime number bigger than 3, then p^2-1 is always divisible by 24 with no remainder?</p>
2013-01-08T23:27:59Ztag:www.mindcipher.com,2005:Puzzle/222013-01-08T23:25:21Z2013-09-30T16:01:37Z/puzzles/22Greatest Area
<p>A farmer challenges an engineer, a physicist, and a mathematician to fence off the largest amount of area using the least amount of fence.</p>
<p>The engineer made his fence in a circle and said it was the most efficient.</p>
<p>The physicist made a long line and said that the length was infinite. Then he said that fencing half of the Earth was the best.</p>
<p>The mathematician laughed at the others and with his design, beat the others. What did he do?</p>
2013-01-08T23:25:21Ztag:www.mindcipher.com,2005:Puzzle/212013-01-08T23:23:45Z2014-02-22T21:01:43Z/puzzles/21Mutilated Chessboard Problem
<p>Suppose a standard 8×8 chessboard has two diagonally opposite corners removed, leaving 62 squares. Is it possible to place 31 dominoes of size 2×1 so as to cover all of these squares?</p>
2013-01-08T23:23:45Ztag:www.mindcipher.com,2005:Puzzle/202013-01-08T23:21:46Z2013-01-26T23:24:30Z/puzzles/20Crazy Google Interview Question
<p>You are shrunk to the height of a nickel and your mass is proportionally reduced so as to maintain your original density. You are then thrown into an empty glass blender. The blades will start moving in 60 seconds. What do you do?</p>
2013-01-08T23:21:46Ztag:www.mindcipher.com,2005:Puzzle/182013-01-08T23:19:08Z2013-08-20T04:24:29Z/puzzles/18The Bookworm
<p>Looking onto a shelf you see three books each labeled Volume 1, Volume 2, and Volume 3 respectively.</p>
<p>Each book is 2.5 inches thick; that is the pages are 2 inches, and each cover .25 inches.</p>
<p>A bookworm begins eating it’s way from the first page of Volume 1, and stops at the last page of Volume 3.</p>
<p>How many inches did the bookworm travel?
(Note he goes in a straight line and can eat through covers, if need be)</p>
2013-01-08T23:19:08Ztag:www.mindcipher.com,2005:Puzzle/172013-01-08T23:17:56Z2013-08-20T04:27:31Z/puzzles/17The Number Problem
<p>What does 3685=?</p>
<p>8809=6
7111=0
2172=0
6666=4
1111=0
3213=0
7662=2
8096=5
9312=1
4444=0
6824=3
9999=4
0345=1
2359=1
5522=0
9884=5</p>
<p>Enjoy, this is quite interesting.</p>
2013-01-08T23:17:56Ztag:www.mindcipher.com,2005:Puzzle/162013-01-08T23:14:57Z2014-08-25T23:08:43Z/puzzles/16Choose the Next Letter
<p>Choose the next letter in the sequence:</p>
<p>O T T F F S S ?</p>
2013-01-08T23:14:57Ztag:www.mindcipher.com,2005:Puzzle/152013-01-08T23:14:32Z2013-09-08T08:10:06Z/puzzles/153 Brilliant People
<p>There are three genii standing in a row in a room facing the same direction. Thus, the genius at one end can see the two genii ahead of him, the genius in the middle can only see the one genius ahead of him and the genius at the front cannot see anybody.</p>
<p>You tell the genii that you have 5 hats: 3 red hats, and 2 white hats.
You then blindfold them, randomly place three of the hats on their heads and hide the remaining hats. You remove their blindfolds and ask them to say the colour of their hats.</p>
<p>The genius at the back says “I don’t know what colour my hat is.”
The genius in the middle says “I don’t know what colour my hat is.”
The genius at the front says “My hat is ______.”</p>
<p>Fill in the blank.</p>
2013-01-08T23:14:32Ztag:www.mindcipher.com,2005:Puzzle/142013-01-08T23:12:07Z2013-10-08T12:34:12Z/puzzles/14Attic Lamp Switch
<p>A downstairs panel contains three on-off switches. One of them controls the lamp in your attic. Using only ONE trip up to the attic, how do you determine which switch controls the lamp?</p>
2013-01-08T23:12:07Ztag:www.mindcipher.com,2005:Puzzle/132013-01-08T23:10:37Z2014-08-19T06:31:07Z/puzzles/13The North Pole Bear Puzzle Reloaded
<p>A man walks 1 mile south, then 1 mile east, and finally 1 mile north to find himself in exactly the same position as he started. He is NOT at the north pole, nor does he shoot a bear.
Where is he?</p>
<p>Original Credit: Martin Gardner</p>
2013-01-08T23:10:37Ztag:www.mindcipher.com,2005:Puzzle/122013-01-08T23:09:04Z2013-11-18T22:36:02Z/puzzles/12Open Water
<p>A man is in a rowing boat floating on a lake, in the boat he has a brick. He throws the brick over the side of the boat so that it lands in the water. The brick sinks quickly. The question is, as a result of this does the water level in the lake go up or down?</p>
2013-01-08T23:09:04Ztag:www.mindcipher.com,2005:Puzzle/112013-01-08T23:07:38Z2014-08-25T23:07:49Z/puzzles/11Clock Works - Google Interview Question
<p>How many degrees are there in the angle between the hour and minute hands of a clock when the time is a quarter past three?</p>
2013-01-08T23:07:38Ztag:www.mindcipher.com,2005:Puzzle/102013-01-08T23:05:37Z2014-03-06T21:26:02Z/puzzles/10Surgery
<p>A boy and his father get into an enormous and unfortunate car accident. The father is taken to one hospital and the son to another. The boy’s surgeon walks into the hospital room and says “I cannot operate on this boy. He is my son.”
What’s going on?</p>
2013-01-08T23:05:37Ztag:www.mindcipher.com,2005:Puzzle/92013-01-08T23:04:14Z2013-10-01T02:59:45Z/puzzles/9The fake coin
<p>You have 9 coins which all look identical but 1 of them weight less than the others. You have a two pan balance (see the picture) with which you can measure twice, and with this 2 measurements you have to determine which coin is the odd one out.</p>
2013-01-08T23:04:14Ztag:www.mindcipher.com,2005:Puzzle/82013-01-08T23:02:12Z2014-05-10T20:36:51Z/puzzles/8Cards With Crosses
<p>You have a stack of 23 cards. Each card has an image of a cross on one side, and an image of a circle on the other side. You know that 14 cards in the stack are laying with the cross up, and therefore 9 cards are with the circle up. But you don’t know in which order the cards are. Moreover, you are in a completely dark room, so you can’t see anything.</p>
<p>How can you, without seeing anything, divide the stack of cards in two smaller stacks, in such a way that both stacks have the same number of cards with a cross up?</p>
2013-01-08T23:02:12Ztag:www.mindcipher.com,2005:Puzzle/72013-01-08T23:00:10Z2014-03-24T14:51:24Z/puzzles/7The Famous Hole in a Sphere
<p>This incredible problem was first made famous by Martin Gardner with his ‘Mathematical Games’ column published in Scientific American throughout the 60s and 70s.</p>
<p>A cylindrical hole, 6 inches long, has been drilled straight through the centre of a solid sphere. What is the volume remaining in the sphere?</p>
<p>No additional data is needed to solve this. (really!)</p>
<p></p>
2013-01-08T23:00:10Ztag:www.mindcipher.com,2005:Puzzle/62013-01-08T22:58:57Z2013-09-30T15:47:52Z/puzzles/6Don't Eat Me!
<p>Would you rather a crocodile attack you or an alligator?</p>
2013-01-08T22:58:57Ztag:www.mindcipher.com,2005:Puzzle/52013-01-08T22:57:33Z2013-11-15T22:19:19Z/puzzles/5Three IdolsHere's a classic:
Three gods A, B, and C are called, in some order, **True, False**, and **Random**. True always speaks truly, False always speaks falsely, but whether Random speaks truly or falsely is a completely random matter.
Your task is to determine the identities of A, B, and C by asking three yes-no questions; each question must be put to exactly one god. The gods understand English, but will answer all questions in their own language, in which the words for yes and no are *‘da’* and *‘ja’*, in some order. You do not know which word means which.
2013-01-08T22:57:33Ztag:www.mindcipher.com,2005:Puzzle/42013-01-08T22:55:49Z2013-12-31T10:13:37Z/puzzles/4The bus stop
<p>You’re driving your car to work in the pouring rain when you see three people on the side of the road at an uncovered bus stop. You recognize one as your best friend waiting to go to work, the second is you perfect lover someone that if you met you would eventually get married to, and the third is an elderly woman soaked to the bone, shivering and needs to go to the hospital.</p>
<p>The problem is that you only have one extra seat in your car so who do you give a ride to?</p>
2013-01-08T22:55:49Ztag:www.mindcipher.com,2005:Puzzle/32013-01-08T22:54:27Z2014-01-27T22:10:56Z/puzzles/33 Men and a Hotel Room
<p>Three men decided to split the cost of a hotel room. The hotel manager gave them a price of $30. The men split the bill evenly, each paying $10, and retired to their room.</p>
<p>However, the manager realized that it was a Wednesday night, which meant the hotel had a special: rooms were only $25. He had overcharged them $5!</p>
<p>He promptly called the bellboy, gave him five one-dollar bills and told him to return it to the men. When the bellboy explained the situation to the men, they were so pleased at the honesty of the establishment that they promptly tipped the bellboy $2 of the $5 he had returned and each kept $1 for himself.</p>
<p>The Problem: Each of the three men ended up paying $9 (their original $10, minus $1 back) totalling $27, plus $2 for the bellboy makes $29. Where did the extra dollar go?</p>
2013-01-08T22:54:27Ztag:www.mindcipher.com,2005:Puzzle/22013-01-08T22:51:44Z2013-04-17T03:09:43Z/puzzles/2Psycho Killer
<p>While at the funeral of her mother, she met a man whom she did not know. She thought he was amazing, her dream guy, and she fell in love with him but never asked for his number and could not find him after the funeral.
A few days later the girl killed her sister.
What was her motive in killing her sister?</p>
2013-01-08T22:51:44Ztag:www.mindcipher.com,2005:Puzzle/12013-01-08T22:51:19Z2014-08-15T12:09:04Z/puzzles/1Burning Rope
<p>There are two lengths of rope.
Each one can burn in exactly one hour.
They are not necessarily of the same length or width as each other.
They also are not of uniform width (may be wider in middle than on the end), thus burning half of the rope is not necessarily 1/2 hour.</p>
<p>By burning the ropes, how do you measure exactly 45 minutes worth of time?</p>
2013-01-08T22:51:19Ztag:www.mindcipher.com,2005:Puzzle/512013-01-08T22:48:02Z2014-03-06T21:23:27Z/puzzles/51What am I?The first person makes me and sells me.
The second person buys me but doesnt use me.
The third person uses me but doesnt see me.
What am I?2013-01-08T22:48:02Z