排列组合中的顺序问题

tigercsh

5个点（其中有一红点）排成一个圆圈，5个人A、B、C、D、E，其中A必须站在红点上，问有多少种不同的站法。
[确认]: P(4,4)/2 = 24/2=12
？？？？为什么要除2？如何通过顺序区别排列和组合？
There are 5 cars, 3 in red, 1 in blue and 1 in white. If all the cars are the same except the color, and now they are to be arranged facing the wall. There are how many ways to arrange them?
[确认]: P5/P3=20
？？？？为什么要除3！

谢谢！

tocean0222

1、the answer should be 24;
2、because the 3 red cars are identical;

donna

以下是引用tocean0222在2003-7-16 15:16:00的发言：
1、the answer should be 24;
2、because the 3 red cars are identical;

第二题GG写错了,because the 3 red cars are not identical.不可分辨,而非可分辨哦!少写了一个NOT.

tocean0222

以下是引用donna在2003-7-16 20:43:00的发言：

第二题GG写错了,because the 3 red cars are not identical.不可分辨,而非可分辨哦!少写了一个NOT.

mm,"identical" means the same, not "identifiable".

donna

以下是引用tocean0222在2003-7-16 22:16:00的发言：

mm,"identical" means the same, not "identifiable".

天啦!我都没注意是identical,哎!又犯错误了.