DAY 421

HEADS OR TAILS

You are in a dark room and have 100 coins spread out in front of you. Ninety are facing heads up and ten are facing tails up. The coins are mixed up, and you cannot see which way they are facing. How can you sort the coins into two piles, such that each pile has the same number of coins facing tails up?

Answer:

Since the piles don't need to be the same size, make one pile of ten coins and one pile of the other ninety. Flip all of the coins in the pile of ten, and now both piles will have the same number of coins facing tails up.

Example:
Suppose you have three coins facing tails up in the pile of ten. This means there are seven coins facing tails up in the pile of ninety. If you flip all ten coins in the ten pile, you now have seven coins facing tails up - just as in the pile of ninety.