原文如下
Ten children are to be divided into an A team and a B team of 5 each. The A team
will play in one league and the B team in another. How many different divisions are
possible?
（翻译：10个孩子分成A,B两队，每队5个人，两队之间做对抗，有多少种分法？）
Solution. There are10!/（5! 5!）= 252 possible divisions.
第二题是
In order to play a game of basketball, 10 children at a playground divide themselves
into two teams of 5 each. How many different divisions are possible?
（翻译：10个孩子分成2个对打篮球，每队5个人，有多少种分法？）
Solution. Note that this example is different from Example 5b because now the order
of the two teams is irrelevant. That is, there is no A and B team, but just a division
consisting of 2 groups of 5 each. Hence, the desired answer is
（翻译：注意这个例子和上个例子不同的地方是这两个队互不相关，没有A,B两队，只分成2个各包含5人的组）
(10!/(5! 5!))/2!= 126 .
第一题和第二题为什么不同？？
我没看懂，不都是分成两组吗？一组5个人。
拜托各位了～
比如【A队成员1,2,3,4,5号，B队成员6,7,8,9,10号】
与【B队成员1,2,3,4,5号，A队成员6,7,8,9,10号】属于2种不同分法
而第二题上面的分法看成是重复的分法，也就是属同一种
