###Human: 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?, ### Assistant: Stanley sold 4 x 3 = 12 cups
Carl sold 7 x 3 = 21 cups
Carl sold 21 - 12 = 9 cups more than Stanley 
 So the answer is :  9