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

### Instruction:
 莎拉每个月都给她在中国的朋友寄信。她在一月寄了6封信，在二月寄了9封信，三月她寄了比一月的信件数量多三倍。莎拉一共寄了多少封信？


### Response:
 由于三月有31天，莎拉在三月寄了3 * 6 = <<3*6=18>>18封信。
总共她寄了6 + 9 + 18 = <<6+9+18=33>>33封信。