诚心求教一道prep PS题目

ゞ綵婲尐ㄝ且 · 发表于 2010-8-23 22:29:54

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
E1/15
答案是A。。。

这竟然是我在CD上的处女贴。。啊啊啊~~~望好心人解释~谢谢

wenjie87 · 发表于 2010-8-23 22:32:45

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
E1/15
答案是A。。。

这竟然是我在CD上的处女贴。。啊啊啊~~~望好心人解释~谢谢

-- by 会员ゞ綵婲尐ㄝ且 (2010/8/23 22:29:54)

C(1,60)/[C(1,1000)*C(1,800)]=3/4000

modernpl · 发表于 2010-8-23 22:40:25

听XDF的解释好像是

先在1000个人里面选，既然有60对双胞胎，那么选中的概率是60/1000

然后因为1000中的那个双胞胎之一确定了以后 800人中的另外一个双胞胎也就随之确定了所以是1/800

再把这两个数相乘

森然 · 发表于 2010-8-23 22:45:45

(60/1000)*(1/800)=3/40000~~

gesteven · 发表于 2011-2-19 01:26:25

哦，就是说配好对的，一个选了，另一个就自动确认了……谢谢

诚心求教一道prep PS题目

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