求助~~~~~~一道概率题，怎么做啊。。。。==

zhangzhenggmat · 发表于 2012-3-11 10:41:34

A certain basket contains 10 apples, 3 of which are green and 7 of them are red. If three different apples are selected at random from the basket, what is the probability that 2 of them are red and 1 of them is green?答案是21/40。。。。= =怎么算都不是这个数啊。。。。求大神详解。。。。。。

michelleqi · 发表于 2012-3-11 11:24:38

【C（7在下，2在上）*C（3在下，1在上）】/C（10在下，3在上）=21/40
因为没法写，只能表达成这样了···

求助~~~~~~一道概率题，怎么做啊。。。。==

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