PS排列组合一道求教各位大牛！！！

dawntouch · 发表于 2012-3-3 22:47:19

Five red balls and two blue balls are placed in an empty box. One ball a time is to be selected randomly and removed 4 balls from the box. What is the probability that the first two balls removed will be red and the third ball removed will be blue? 求思路

treewing929 · 发表于 2012-3-3 22:54:44

我估: 1R2R3B4R or 1R2R3B4B = 5/7 *4/6 *2/5 * 3/4 + 5/7* 4/6* 2/5* 1/4

?

dawntouch · 发表于 2012-3-4 14:57:50

是的答案算出来是4/21，谢谢。

elvaxiaobai · 发表于 2012-3-4 15:43:29

5红2蓝
5/7*4/6*2/5*1 因为最后一个不管什么色都行

PS排列组合一道求教各位大牛！！！

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