Stanley sold 4 cups of lemonade an hour. His brother, Carl, sold 7 cups of lemonade an hour. How many more cups did Carl sell than Stanley in 3 hours? Let's answer step by step:
Stanley sold 4 x 3 = 12 cups. Carl sold 7 x 3 = 21 cups. Carl sold 21 - 12 = 9 cups more than Stanley. The answer: 9.