Sort by:
HardestAll puzzles
|
5
5
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
|
|
6
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
...
|
Hard
|
|
8
5
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
...
|
Hard
|
|
7
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?
|
Hard
|
|
11
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
...
|
Hard
|
|
2
2
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
|
|
3
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
...
|
Hard
|
|
6
2
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
...
|
Hard
|
|
2
1
Open Water
A man is in a rowing boat floating on a lake, in the boat he has a brick. He throws the brick over the side of the boat so that it lands in the water. The brick sinks quickly. The question is, as a r
...
Submitted by Valentin
|
Hard
|
|
1
4
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
...
|
Hard
|
|
2
4
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
|
|
5
5
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
...
|
Hard
|
|
1
Cards With Crosses
You have a stack of 23 cards. Each card has an image of a cross on one side, and an image of a circle on the other side. You know that 14 cards in the stack are laying with the cross up, and therefor
...
Submitted by Valentin
|
Hard
|
|
2
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
|
|
1
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
|