请大虾出手相助 og11-217

lansha8uo

217: A certain juniour 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?

答案是  A )  3/40,000

     我选c)9/2,000

我的思路是这样的:60/1000*60/800

答案思路是60/1000*1/800

我不理解, 请大虾出手相助?

Aristotle

1.about the 60/1000.

It means that the probability of a student in the junior class whom has a sibling is 60/1000. Only under this situation can he or she finds his or her elder sibling in the senior class.

2.about the 1/800

the junior student was chosen by 60/1000. So next step is finding his or her sibling. Everybody in the senior class has the same probability.

So the final answer is 60/1000 * 1/800

xiaoxiaoqing

（60/1000）*（1/800）=3/40,000 先从juniour class选，其中有60人在sibling pairs中，所以是60/1000，接下来在senior class中只能选对应的那个人，所以是1/800

lansha8uo

 很清晰, thanks a lot , i got it