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

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 5 Birthday Problem (Classic) A classic problem. 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%. Assume 365 days in a year (no leap years) ... Submitted by Valentin Hard 8.4 13 Black and White Hats There 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 ... Submitted by Aakash Hard 8.5 8 Prisoners and hats 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 freel ... Submitted by Valentin Hard 8.5 8 Survivor There 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 s ... Submitted by Shaheen Hard 8.5 6 Partitioning a set In 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 [Partit ... Hard 8.7 8 Non-Adaptive 12 Identical Balls Okay - so I posted this puzzle to reddit a few weeks ago and one guy had the following comment: Same problem, but your solution must be non-adaptive. IOW, specify the weighings that you will make up f ... Submitted by Aakash Hard 9.0 9 Three Idols Here'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 comp ... Submitted by Aakash Hard 9.1 1 interesting visual puzzle I've made this puzzle myself so I don't know if it's fair Hint: you might need a pencil and paper and look at the problem through a different perspective Submitted by soni Hard 10.0 4 No if() no abs()! This is inspired by the puzzle "No 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 +- ยท / (basic arithmetic operator ... Submitted by Doodz Hard 10.0