SAT数学,有关排列组合

题目

SAT数学,有关排列组合
in a certain soccere league, each of the five teams plays every other team in the league exactly three times each season. How many games are played in total in one season?
答案是30.为什么?

答案

用最普通的方法想,每个队和其他4个队各比一次,两队之间又是重复的,就是（5*4）/2次,再乘3就是总次数~
其实用公式很简单,不过我也忘得差不多了,也不知道那符号怎么打出来……总之祝考试成功~