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

### Instruction:
 莫正在为班级购买情人节卡片。班上有30名学生，他想给其中60%的学生送情人节卡片。每张卡片的价格是2美元。如果他有40美元，他将花费多少比例的钱购买情人节卡片？


### Response:
 30 x 60% = <<30*60*.01=18>>18名学生
18 x 2 = $<<18*2=36>>36
36 / 40 = 他花费的收入的90%