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?
A step-by-step solution is:
9
Stanley sold 4 x 3 = 12 cups.

Carl sold 7 x 3 = 21 cups.

Carl sold 21 - 12 = 9 cups more than Stanley.

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So, the answer is 9