Explain simply why 3 is the correct answer to: Bret takes a 9 hour train ride to go to Boston.  He spends 2 hours reading a book, 1 hour to eat his dinner, and 3 hours watching movies on his computer. How many hours does he have left to take a nap?. Explanation:
For all his activities on the train, Bret spends 2 + 1 + 3 = 6 hours. For his nap, he has 9 - 6 = 3 hours left.