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

### Instruction:
 如果你购买超过15个iPhone手机壳，你可以将价格降低20%。如果你支付$500购买18个单位，原价是多少？


### Response:
 如果你购买18个单位，总价格的百分比降低到100% - 20% = 80%
如果80%等于$500，那么100%等于100%/80% * $500 = $625