Problem 2. We have two hourglasses that measure 7 minutes and 11 minutes. An egg needs to boil for 15 minutes. How can we measure this time using the two sand hourglasses?
Solution. I will start both hourglasses at the same time. When the 7-minute hourglass runs out, I will start boiling the egg. The 11-minute hourglass still has 4 minutes left to run out. After it runs out, I will immediately start it again to measure 11 minutes. Since $4+11=15$, after it runs out, the egg will be boiled.