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

### Instruction:
 有一群18个人要订披萨。如果每个人吃3片，每个披萨有9片，他们应该订购多少个披萨？


### Response:
 每个人吃3片披萨，整个群体吃了18 x 3 = <<18*3=54>>54片。
54片披萨需要54 / 9 = 6个。