Below is an instruction that describes a task. Write a response that appropriately completes the request in Chinese. Please answer in Chinese.

### Instruction:
 斯坦利每小时卖4杯柠檬水。他的弟弟卡尔每小时卖7杯柠檬水。卡尔在3小时内比斯坦利多卖了多少杯？


### Response:
 斯坦利卖了4 x 3 = <<4*3=12>>12杯。
卡尔卖了7 x 3 = <<7*3=21>>21杯。
卡尔比斯坦利多卖了21 - 12 = <<21-12=9>>9杯。