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

### Instruction:
 4个朋友去服装店购物。每件商品都打5折。四个朋友决定买一件T恤。T恤的原价是20美元。他们总共花了多少钱？


### Response:
 经过50%的折扣，T恤的价格是20 * 0.5 = $<<20*0.5=10>>10。
所以，四个朋友总共花了4 * $10 = $<<4*10=40>>40。