Welcome to MindCipher, a social repository of the world's greatest brain teasers, logic puzzles and mental challenges.
16 3 100 Light Bulbs
There 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 on ...
mathematical logical Submitted by Aakash
Hard
7.6
5 1 Count The Bits
Write 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.
Hard
7.6
56 62 All the King's Wine
So 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 ...
logic mathematical Submitted by Aakash
Hard
7.5
6 3 Clever Commuter
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 ...
Submitted by Valentin
Hard
7.5
7 7 The Neighbourhood
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: There are 5 houses each with a different color. Th ...
Hard
7.5
1 2 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.
riddle funny Submitted by Ravi
Hard
7.3
4 3 Up and Down
Explain the following scenario: 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 int ...
Submitted by Valentin
Hard
7.3
2 3 Lockers
A high school has a strange principal. On the first day, he has his students perform an odd opening day ceremony: There are one thousand lockers and one thousand students in the school. The princip ...
Submitted by Aakash
Hard
7.1
4 3 The Price Is Right
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 th ...
Submitted by Aakash
Hard
7.0
2 13 Good 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 ...
google open-ended Submitted by Valentin
Hard
7.0
2 3 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 ...
Submitted by Ravi
Hard
7.0
5 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 ...
Submitted by Ravi
Hard
7.0
4 2 What 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?
Submitted by Valentin
Hard
6.8
4 4 Left Hands Only
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? For t ...
Submitted by Aakash
Hard
6.8
8 15 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, ...
Math Lateral geometry Submitted by Ravi
Hard
6.7