Puzzles: Set 1
Set 1
Set 2
Set 3
Set 4
Set 5
Set 6
Set 7
Set 8
Chameleons

On an island live 13 purple, 15 yellow and 17 maroon chameleons. When two chameleons of different colors meet, they both change into the third color. Is there a sequence of pairwise meetings after which all chameleons have the same color?
Solution
Four Pills

A blind man has four pills: two red and two blue. The pills are identical in tehrms of shape, size, texture, weight, smell and so on. Only the colors are different. So the blind man cannot distinguish between a red pill and a blue pill. Now the blind man must consume exactly 1 red pill and 1 blue pill. If he consumes two red pills or two blue pills, he will immediately die. What strategy will enable the blind man to ensure that he consumes exactly one pill of each color?
Solution
Pebble Piles

You are given three piles with 5, 49 and 51 pebbles respectively. Two operations are allowed: (a) merge two piles together or (b) divide a pile with an even number of pebbles into two equal piles. Is there a sequence of operations that would result in 105 piles with one pebble each?
Solution
Rope Escape

Rajeev is trapped atop a building 200m high. He has with him a rope 150m long. There is a hook at the top where he stands. Looking down, he notices that midway between him and the ground, at a height of 100m, there is a ledge with another hook. In his pocket lies a Swiss knife. Hmm... how might he be able to come down using the rope, the two hooks and the Swiss knife?
Solution
Cake Cutting

Mary baked a rectangular cake. Merlin secretly carved out a small rectangular piece, ate it and vanished! The remaining cake has to be split evenly between Mary's two kids. How could this be done with only one cut through the cake?
Solution
Fox in a Hole

Consider five holes in a line. One of them is occupied by a fox. Each night, the fox moves to a neighboring hole, either to the left or to the right. Each morning, you get to inspect a hole of your choice. What strategy would ensure that the fox is eventually caught?
Solution
Cube Cutting

Imagine a 3x3x3 wooden cube. How many cuts do we need to break it into twenty-seven 1x1x1 cubes? A cut may go through multiple wooden pieces.
Solution
Ant Collisions

Peter and Cynthia stand at each end of a straight line segment. Peter sends 50 ants towards Cynthia, one after another. Cynthia sends 20 ants towards Peter. All ants travel along the straight line segment. Whenever two ants collide, they simply bounce back and start traveling in the opposite direction. How many ants reach Peter and how many reach Cynthia? How many ant collisions take place?
Solution
Ant in a Room

An ant crawls from one corner of a room to the diametrically opposite corner along the shortest possible path. If the dimensions of the room are 3 x 4 x 5, what distance does the ant cover?
Solution
Coin Toss Guess

Alice and Bob are playing a game. They are teammates, so they will win or lose together. Before the game starts, they can talk to each other and agree on a strategy.

When the game starts, Alice and Bob go into separate soundproof rooms — they cannot communicate with each other in any way. They each flip a coin and note whether it came up Heads or Tails. (No funny business allowed — it has to be an honest coin flip and they have to tell the truth later about how it came out.) Now Alice writes down a guess as to the result of Bob’s coin flip; and Bob likewise writes down a guess as to Alice’s flip.

If either or both of the written-down guesses turns out to be correct, then Alice and Bob both win as a team. But if both written-down guesses are wrong, then they both lose.

Can you think of a strategy Alice and Bob can use that is guaranteed to win every time?

Solution