作者catspace (柏拉图的永恒...)
看板GMAT
标题Re: [计量] PP-T1-PS116
时间Thu Sep 25 13:55:55 2008
※ 引述《Earadriewiel (ㄣ)》之铭言：
: A certain junior class has 1,000 students and a certain senior class has 800
: students. Among these students, there are 60 sibling pairs, each consisting of
: 1 junior and 1 senior. If 1 student is to be selected at random from each
: class, what is the probability that the 2 students selected will be a sibling
: pair?
: (A)3/40000
: (B)1/3600
: (C)9/2000
: (D)1/60
: (E)1/15
: 答案给A，请问一下有人可以说明一下算法吗 ?...感谢搂
60 3
P = -------- = -------
1000*800 40000
分母是两个班级各选一个的可能数
分子是双胞胎的可能数
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1F：推 Earadriewiel:谢谢喔! 09/25 14:02