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

### Ravi

#### Likes 89

 2 What Am I? 12 Choose the Next Letter 3 The fake coin 7 The bus stop 9 Three Idols 18 3 Men and a Hotel Room 3 What number is next? 10 Mutilated Chessboard Problem 8 Attic Lamp Switch 4 What am I? Show More
 5 Betsy and Patsy 4 Time To Fuse 2 The North Pole Bear Puzzle Reloaded 4 Balloon Business 54 All the King's Wine 1 Right or Left? 8 Prisoners and hats 5 Balloon in an Elevator 3 Are the lines parallel? 6 The Female Star 8 All Apples 8 Interview Question - Databases 3 The Cabin 4 Gone Camping 16 100 Light Bulbs 18 Cocktail Party 19 Going to the Track 27 12 Identical Balls 12 Carpeting a Donut 4 Boys and Girls 5 Clock Hands - Google Interview Question 6 Salary 9 Unfair Coin 4 Electrical Deaths 10 Fork in the road 1 Running Race 10 3 Coins 15 The double-square number problem 4 Don't Hang Yourself 11 Measuring Up 3 Products Algorithm 13 Bit Pair Swap 45 The Sinking Island 5 Balloon in an Elevator 10 3 Coins 9 Unfair Coin 13 Bit Pair Swap 45 The Sinking Island 3 Power of 2 (Microsoft Interview) 10 Two Trains 9 The Monty Hall Problem 15 Swap Variables in Place without XOR 37 No if() Only abs() 10 Sum of Two Prime Numbers 11 Gold or silver? 4 Electrical Deaths 10 Two Trains 9 The Monty Hall Problem 14 Coin Flip 8 Non-Adaptive 12 Identical Balls 5 Crazy Google Interview Question 6 Grey Squares Illusion 17 Multiples of 8 plus 1 20 Odd Pills 6 Find the Missing Number 7 The Neighbourhood 8 The Famous Hole in a Sphere 6 Partitioning a set 5 Matchsticks 12 Simple 24 5 Troll Toll 9 1978 4 Don't Mate in One 46 Charlie and the Chocolate Factory 8 Survivor 6 Binary 5 digits 2 How far can you go? 4 Cut the pizza into 11 equal slices with a wrist watch 5 Count The Bits 13 Black and White Hats 5 Lab Rat on the Run 6 Death by drawing? 3 How many? 3 Brain Teaser 56 4 There are only 4 circles and none of them touch 5 A Pharmacist's Nightmare 4 Sold! 5 Sand Timer Puzzle 5 Age of Three Kids

#### Ratings 63

 12 2.0 Choose the Next Letter 3 6.0 What number is next? 4 10.0 What am I? 10 10.0 Mutilated Chessboard Problem 6 2.0 Psycho Killer 8 10.0 Attic Lamp Switch 5 6.0 Betsy and Patsy 4 10.0 Time To Fuse 6 2.0 Find the Missing Number 18 2.0 3 Men and a Hotel Room Show More
 12 2 Choose the Next Letter 3 6 What number is next? 4 10 What am I? 10 10 Mutilated Chessboard Problem 6 2 Psycho Killer 8 10 Attic Lamp Switch 5 6 Betsy and Patsy 4 10 Time To Fuse 6 2 Find the Missing Number 18 2 3 Men and a Hotel Room

 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. context 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. context 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. context All the King's Wine Can you elaborate on your solution? context 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? context All the King's Wine You're right. Perhaps the original intention was to say "each of the servants can/must try multiple wines"? context 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) context Age of Three Kids Solid puzzle. context Don't Mate in One A picture has been added for clarity. context Sand Timer Puzzle Sakshi -- no spamming. context 1978 +1 to Ryan Miller's comment. context Gold or silver? I guess this one falls more into the "riddle" category than "chemistry". context 3 Coins What's your reasoning alvinfromva? context 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. context 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)}] context 1978 Spoiler tags were recently implemented so that you can spoilerhide parts of your comments! context Electrical Deaths I agree. There's quite a few loopholes in the solution, but it's a fun problem nonetheless. context 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? context 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? context The Cabin That's a great point. We're always brainstorming new features so we'll definitely keep this in mind! context The Sinking Island Most of the puzzles on here have solutions. Click Show Answer and Show Solution. context 12 Identical Balls I agree. I assume it means "identical in appearance". context 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. context Gold or silver? Touché. Clever observation. context Ball Drop I agree. I'll assume the golf ball would encounter less drag, and would therefore hit the sea floor first. context Survivor I knew it looked familiar! context 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). context Sum of Two Prime Numbers There's a Show Answer button below the puzzle! context Carpeting a Donut I agree, that's a lot more intuitive than the OP's solution. context 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`. context 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? context Find the Missing Number Ah, very clever solution! Have you heard this problem before or did you figure that out by observation? context Coin Flip Wow, you even went as far as the Monte Carlo simulation. Impressive. context 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. context Charlie and the Chocolate Factory Urvesh, there's a Show Solution button to check if your solution is correct! context Multiples of 8 plus 1 Agreed, great solution! context The Bookworm +1 (3 x 2.5") - 0.25" for the front cover of Vol. 1 - 0.25" for the back cover of Vol. 3. context Swap Variables in Place without XOR Yep, that's exactly the solution provided. context 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. context 1978 Well played, good sir. context 12 Identical Balls There's also the 9 ball version with 2 weighs. context Tooth Paste, : Profit I think the solution here rests on some assumptions that aren't clear in the question: * What does the mouth diameter have to do with the actual volume of toothpaste in the tube? Wouldn't this just be compensated for by just squeezing the toothpaste tube less or more? * Where is the 10, 2 and 3 days coming from? If I use 1g per day from the 100g toothpaste, it'll last me 100 days :) context 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. context Death by drawing? Yeah, the math is a bit messy, but you're finding the max of this. I'm not sure how to treat the b and w variables as whole numbers in Wolfram Alpha, but under the assumption that w > 0, where w is the number of marbles in the first bag, you're answer of 73.7% makes sense. I think this should solve it, but free Wolfram Alpha is telling me I exceeded standard computation time and need to pay for more :(. context No if() Only abs() That's an awesome explanation. MoralMachine, I suggest adding it to the puzzle solution. context