Welcome to MindCipher, a social repository of the world's greatest brain teasers, logic puzzles and mental challenges.
15 14 Swap Variables in Place without XOR
Suppose you have two variables, a and b. How can you swap their values without using a temporary variable and also without using the bitwise operator XOR?
Doable
3.9
13 16 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
6.6
1 4 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
15 10 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
8.1
10 22 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
12 7 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
5.5
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
6 10 Binary 5 digits
What is the base 10 equivalent of the highest 5 digit binary number?
Easy
2.0
18 4 3 Men and a Hotel Room
Three men decided to split the cost of a hotel room. The hotel manager gave them a price of $30. The men split the bill evenly, each paying $10, and retired to their room. However, the manager real ...
Submitted by Valentin
Doable
4.2
8 10 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
5 4 Sand Timer Puzzle
You have two sand timers with you. One can measure 7 minutes and the other sand timer can measure 11 minutes. This means that it takes 7 minutes for the sand timer to completely empty the sand from one ...
Submitted by Rohit Gurjar
Doable
5.0
9 12 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
6.6
10 8 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
Easy
3.2
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
3 7 100 kg bag of potatoes
if you have a 100kg bag of potatoes, that are 99% water, they're left out in the sun and dry up a bit, so it's now 98% water, how heavy is the bag of potatoes?
Submitted by chuck3r
Doable
5.0