请教gwd 一道数学题

In a meeting of 3 representatives from each of 6 different companies,each person shook hands with every person not from his or her own company.If the representatives did not shake hands with people from their own company , how many handshakes took place?

key :135 怎么算的

C18 2 - （C 3 2） *6

一共18个人，每3人来自一个公司，相互之间握手，一个公司内部的不握，问一共有多少次握手发生？

先计算任意两个人握手的次数，是C 18 2，再减去每个公司内部握手的情况，一个公司内部是是C 3 2，乘以6再