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: The piles don't need to be the same size, so make a pile of 10 coins and a pile of 90 coins, flip all of the ten coins and it is guaranteed that the piles have the same number of tails.
Question: When Tommy was 10 years old his sister Suzy was half his age. Now that Tommy is 50 years old how old is his sister Suzy?
Answer: 45 years old. Suzy was 5 when Tommy was 10 so she is 5 years younger.