Suppose you would like to boil an egg for exactly 15 minutes. But you only have two hourglasses, one of zeven and one of eleven minutes. What is the the fastest way to do this?

Turn both hourglasses upside down and put the egg in the water. After 7 minutes, turn the 7-minutes hourglass. Then when 11 minutes have passed, turn the 7-minutes hourglass again. Then it will take another 4 minutes until the 7-minutes hourglass stops. At that time exaclty 15 minutes have passed.