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

### Instruction:
 如果一辆车周一行驶了12英里，周二行驶了18英里，周三行驶了21英里。每天平均行驶多少英里？


### Response:
 从周一到周三的总行驶距离是12 + 18 + 21 = <<12+18+21=51>>51英里。
所以每天平均行驶距离是51/3 = <<51/3=17>>17英里。