Welcome to MindCipher, a social repository of the world's greatest brain teasers, logic puzzles and mental challenges.
15 6 Going to the Track
The London racetrack needs to submit its top three horses to the Kentucky Derby next month in order to compete for a prize. However in a recent electrical storm, all the racetrack's previous race histo ...
logic mathematics Submitted by Nikhil
Doable
6.3
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 6 Lab Rat on the Run
Consider a lab rat in a maze. There are four paths that can be taken with equal probability. Path A leads to exit in 5 minutes. Path B leads back to start and takes 8 minutes. Path C leads to exit in 3 ...
Doable
6.0
7 6 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
8.0
5 5 Binary 5 digits
What is the base 10 equivalent of the highest 5 digit binary number?
Easy
2.0
1 How many?2
How many people contribute to MindCipher?
Submitted by Gene
Easy
2.0
9 6 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 ...
Math mathematics Submitted by Guilherme
Hard
8.2
9 14 3 Coins
Not sure what the answer is, ideas welcome! You have 3 coins, one always comes up heads, the second always comes up tails, and the third is a fair coin. You select a coin at random. After selecting ...
Math help Submitted by city_slick
Easy
3.0
2 4 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?
infinity Submitted by Tarun245
Hard
8.0
9 4 Carpeting a Donut
Given two concentric circles and a chord of the outer circle, tangent to the inner circle, of length 25m, find the area between the two circles. See illustration. No other information needed, all you n ...
Doable
5.7
14 100 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
7.7
9 9 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
8 8 1978
The year 1978 is such that the sum of the first two digits and the latter two digits is equal to the middle two digits, i.e. 19 + 78 = 97. What is the next year (after 1978) for which this is true?
Doable
6.0
9 5 Sum of Two Prime Numbers
If p, q > 2 are consecutive in set of primes. Since p,q can only be odd number, (p+q) is an even number. Can (p+q)/2 be prime? from http://republicofmath.com/
Submitted by MoralMachine
Doable
3.3
9 5 Fork in the road
You are walking down a road and arrive at a fork that splits the road into two separate paths. When you reach, you meet two identical twins, each standing at the edge of one of the two paths. You h ...
Submitted by Aakash
Doable
6.4