The Candy Shop

A candy shop lets children exchange 3 chocolate wrappers for a brand new chocolate. Willy is walking around town collecting chocolate wrappers from people. How many wrappers must he collect in order to eat 10 chocolates?

He needs only 21 wrappers. He uses them to get 7 chocolates. After he eats them, he is left with 7 wrappers, which uses to get 2 more chocolates. After he eats these, he is left with 3 wrappers, which are enough to get one more chocolate and make them 10 in total.