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?
Stanley sold 4 x 3 = <<4*3=12>>12 cups
Carl sold 7 x 3 = <<7*3=21>>21 cups
Carl sold 21 - 12 = <<21-12=9>>9 cups more than Stanley
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