Welcome to MindCipher, a social repository of the world's greatest brain teasers, logic puzzles and mental challenges.

Ravi

Likes 81

2 What Am I?
11 Choose the Next Letter
3 The fake coin
6 The bus stop
8 Three Idols
16 3 Men and a Hotel Room
3 What number is next?
8 Mutilated Chessboard Problem
6 Attic Lamp Switch
3 What am I?
Show More
5 Betsy and Patsy
2 Time To Fuse
1 The North Pole Bear Puzzle Reloaded
4 Balloon Business
39 All the King's Wine
1 Right or Left?
7 Prisoners and hats
4 Balloon in an Elevator
2 Are the lines parallel?
3 The Female Star
8 All Apples
6 Interview Question - Databases
2 The Cabin
3 Gone Camping
13 100 Light Bulbs
16 Cocktail Party
14 Going to the Track
22 12 Identical Balls
9 Carpeting a Donut
4 Boys and Girls
4 Clock Hands - Google Interview Question
5 Salary
8 Unfair Coin
4 Electrical Deaths
9 Fork in the road
1 Running Race
9 3 Coins
8 The double-square number problem
4 Don't Hang Yourself
7 Measuring Up
3 Products Algorithm
11 Bit Pair Swap
28 The Sinking Island
4 Balloon in an Elevator
9 3 Coins
8 Unfair Coin
11 Bit Pair Swap
28 The Sinking Island
3 Power of 2 (Microsoft Interview)
9 Two Trains
9 The Monty Hall Problem
13 Swap Variables in Place without XOR
28 No if() Only abs()
9 Sum of Two Prime Numbers
8 Gold or silver?
4 Electrical Deaths
9 Two Trains
9 The Monty Hall Problem
11 Coin Flip
6 Non-Adaptive 12 Identical Balls
5 Crazy Google Interview Question
5 Grey Squares Illusion
13 Multiples of 8 plus 1
16 Odd Pills
3 Find the Missing Number
5 The Neighbourhood
7 The Famous Hole in a Sphere
5 Partitioning a set
5 Matchsticks
8 Simple 24
4 Troll Toll
7 1978
3 Don't Mate in One
24 Charlie and the Chocolate Factory
6 Survivor
4 Binary 5 digits
2 How far can you go?
2 Cut the pizza into 11 equal slices with a wrist watch
4 Count The Bits
6 Black and White Hats
5 Lab Rat on the Run

Comments 40

12 Identical Balls

There's also the 9 ball version with 2 weighs.

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Don't Mate in One

A picture has been added for clarity.

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1978

+1 to Ryan Miller's comment.

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Gold or silver?

I guess this one falls more into the "riddle" category than "chemistry".

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The Female Star

Most of the puzzles with the "word puzzle" tag may be specific to the English language, but most of the other puzzles are language-independent.

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1978

Spoiler tags were recently implemented so that you can spoilerhide parts of your comments!

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3 Coins

What's your reasoning alvinfromva?

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No if() Only abs()

That's a good point. More verbosely, {a+b+abs(a-b)} / [{a+b+abs(a-b)}+{a+b+abs(a-b)}]

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Electrical Deaths

I agree. There's quite a few loopholes in the solution, but it's a fun problem nonetheless.

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Balloon in an Elevator

In the confines of an elevator shaft where the area of the base of the elevator covers most of the cross-sectional area of the shaft, I would imagine the elevator would reach terminal velocity quite quickly and stop accelerating at G, at which point the balloon would go up. So I guess your answer is assuming an ideal case where the lift keeps accelerating at G?

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Balloon in an Elevator

When the lift is in free-fall, wouldn't it become weightless and therefore its contents would also become weightless? I would think that this would make the balloon drop towards the floor of the elevator. What's your reasoning Timmmm?

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The Cabin

That's a great point. We're always brainstorming new features so we'll definitely keep this in mind!

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The Sinking Island

Most of the puzzles on here have solutions. Click Show Answer and Show Solution.

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12 Identical Balls

I agree. I assume it means "identical in appearance".

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Gold or silver?

If you go down the path of differentiating weight vs mass, you would have to assume that gravity is the same when weighing both elements.

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Gold or silver?

Touché. Clever observation.

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3 Coins

The key is to infer the probability that each coin was selected based on the (heads,heads) outcome for the first two flips.

Label the three coins:

  • C1 - TT
  • C2 - HT
  • C3 - HH

Data = HH. In general, as per Bayes' Rule, P(Ci | Data) is proportional to P(Data | Ci)*P(Ci), so:

  • P(C1 | Data) proportional to 0*(1/3) -----> 0 (normalized)
  • P(C2 | Data) proportional to (1/4)*(1/3) --> 1/5
  • P(C3 | Data) proportional to (1/4)*(1/3) --> 4/5

Now, P(H | Data) = sum over Ci ( P(H | Ci) * P(Ci | Data ) = 0 * 0 + (1/2) * (1/5) + 1 * (4/5) = 9/10.

In summary, probability of heads on the third flip (given the first two flips being HH) is 9/10.

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Ball Drop

I agree. I'll assume the golf ball would encounter less drag, and would therefore hit the sea floor first.

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The Famous Hole in a Sphere

I don't think that's true. The question is asking what the remaining volume of the solid sphere is after the cylinder is removed.

Also, check out this picture. Notice that the height h of the cylinder does not equal the diameter of the sphere.

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Crazy Google Interview Question

Interesting approach! To be fair, if you were the size of a nickel, your shoe laces wouldn't be long enough to wrap around the rotating assembly (unless you had abnormally long shoe laces).

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Survivor

I knew it looked familiar!

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Sum of Two Prime Numbers

There's a Show Answer button below the puzzle!

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Carpeting a Donut

I agree, that's a lot more intuitive than the OP's solution.

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Bit Pair Swap

I don't think that works. For example, if you have 0000 0001, your solution will give you 1111 1110, but the actual solution would be 0000 0010.

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Find the Missing Number

Not sure I follow.

7 4 8

3 9 7

6 5 10

? 8 4

The first and last numbers in each column don't have the same number of letters: ("four","eight"), ("seven, "five")? Also, the question specifies that each row follows the same pattern of numbers. Can you provide an example of your solution?

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Find the Missing Number

Ah, very clever solution! Have you heard this problem before or did you figure that out by observation?

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Coin Flip

Wow, you even went as far as the Monte Carlo simulation. Impressive.

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The double-square number problem

Nice, although brute forcing ruins all the fun! :)

In general, the space of solutions to this problem lie on the circle defined by 100x+y = x^2 + y^2. Here it is on WolframAlpha.

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Multiples of 8 plus 1

Agreed, great solution!

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Charlie and the Chocolate Factory

Urvesh, there's a Show Solution button to check if your solution is correct!

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Swap Variables in Place without XOR

Yep, that's exactly the solution provided.

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Tea and Wine

Hmm, that makes sense. I think the hardest part of this solution is intuitively understanding the duality between tea and wine proportions during the second transfer.

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1978

Well played, good sir.

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All the King's Wine

I was asked this question in a job interview last year. It's a great question that really challenges one's ability to dissect a problem and carefully use the given information. (I'm being intentionally vague so I don't ruin it for others)

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All the King's Wine

You're right. Perhaps the original intention was to say "each of the servants can/must try multiple wines"?

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All the King's Wine

That's hilarious!

And why is the solution you were clearly driving me towards one which takes advantage of an undocumented and unreliable epiphenomenon? Does your team usually write code whose correctness relies upon undocumented and unreliable correlations, correlations whose magnitudes can vary widely as a result of implementation details?

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No if() Only abs()

That's an awesome explanation. MoralMachine, I suggest adding it to the puzzle solution.

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All the King's Wine

Can you elaborate on your solution?

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Lab Rat on the Run

Can you elaborate on your solution? I'm not sure I understand your spoiler(8+t) and (2+t) factors since t is expected time inclusive of any failed attempts.

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All the King's Wine

I would first try to find out if the poison isn't colorless, and if that's the case, you could inspect the color of each wine. It would be especially obvious if it's white wine.

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Submitted Puzzles 28

7 5 The Famous Hole in a Sphere
This incredible problem was first made famous by Martin Gardner with his ‘Mathematical Games’ column published in Scientific American throughout the 60s and 70s. A cylindrical hole, 6 inches long, ...
Hard
6.7
1 2 Cooking Time
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. How can he measure 9 minutes with those clocks?
Doable
6.6
6 4 Bug's Eye
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.
Doable
3.8
4 4 Electrical Deaths
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 ...
Doable
3.3
0 3 Cutting Numbers
When is Seven half of Twelve?
Doable
4.0
3 3 Brilliant People
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 ahea ...
Doable
5.2
3 A Pharmacist's Nightmare
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 p ...
Hard
9.0
3 2 The Stealing Mailman
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 le ...
Doable
6.0
11 13 Bit Pair Swap
In 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.
Doable
6.0
5 1 Grey Squares Illusion
Square A is the exact same shade of gray as square B. It's true, check out the reconstruction.
Doable
6.0
7 3 Tea and Wine
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 ha ...
Doable
4.9
9 8 The Monty Hall Problem
You’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 ...
Doable
5.4
7 2 Measuring Up
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 su ...
Doable
3.3
28 7 The Sinking Island
You 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 ...
Doable
4.4
9 2 Two Trains
Two 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. O ...
Doable
4.7
7 2 Burning Rope
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 th ...
Doable
3.8
1 2 The North Pole Bear Puzzle Reloaded
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? ...
Doable
5.0
2 The Number Problem
What does 3685=? 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 Enjoy, this is quite interesting.
Doable
3.3
8 1 Mutilated Chessboard Problem
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?
Hard
7.1
11 1 Choose the Next Letter
Choose the next letter in the sequence: O T T F F S S ?
Easy
2.7
0 2 3 Painted Ladies
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 wh ...
Doable
6.0
2 2 Nice to meet you
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?
Easy
2.8
1 1 Random Puzzle
Here’s something I heard from a friend, I thought it was appropriate. What do you call a 4 foot tall fortune teller who is wanted by the law? Good luck.
Hard
8.0
1 3 Parodoxical Statements from Life
Nobody goes to that restaurant; it’s too crowded. Don’t go near the water ’til you have learned how to swim. The man who wrote such a stupid sentence can not write at all. If you get this messa ...
Doable
5.2
1 2 Marble Weights
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 c ...
Doable
6.0
8 3 The Tallest Mountain
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 w ...
Easy
2.4
3 7 Find the Missing Number
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. 7 4 8 3 9 7 6 5 10 ? ...
Doable
6.0
0 Classic Knights & Knaves
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. As you a ...
Doable
4.7