How do you sort the coins into two piles that contain the same number of tails up coins?

Question

There are 100 coins scattered in a dark room. 90 have heads facing up and 10 are facing tails up. You cannot tell which coins are which.
How do you sort the coins into two piles that contain the same number of tails up coins?

Answer 1

1. Make a pile of 10 coins and a pile of 90 coins.
   One of the piles have x, the other pile has 10-x tails, where x<=10
2. Flip all of the ten coins.
   The piles now have the same number of tails since 10-(10-x)=x