求助prep的题

nevaeh11 · 发表于 2010-10-7 21:58:31

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?

做的破解版这题没选项。。。
谢谢大家

Jas敏 · 发表于 2010-10-8 00:57:56

60/1000 * 60/800
从junior中抽到sibling pairs中的一个的机率是 60/1000， senior的是60/800

gesteven · 发表于 2011-2-19 01:23:29

楼上算的不对啊，答案是3/40,000

求助prep的题

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