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

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 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 +- · / (basic arithmetic operator ... Submitted by Doodz Hard 10.0 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 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 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 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 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 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 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 16 The double-square number problem It'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 ot ... Submitted by Guilherme Hard 8.1 6 Happy Handshaking 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 ev ... Submitted by Valentin Hard 8.0 2 What is the sum of 1+2+3+4+5 . . . (forever)? What is the sum of 1 + 2 + 3 + 4 . . . all the way to infinity? Submitted by Tarun245 Hard 8.0 1 4 buttons and a light bulb There are two rooms, in one room there is a light bulb, and in the other there is a round table, with 4 buttons arragned in a square on it. Each button may be on or off, and the light bulb will be on o ... Submitted by TulkinRB Hard 8.0 10 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? Submitted by Ravi Hard 7.8 5 Balloon in an Elevator 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 el ... Submitted by Aakash Hard 7.7