求助Prep里面那道sibling的概率题

IShallFly

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?

忘NN指点迷津~~~

honeydd

谢谢，学习了。这题偶也做错了，选2个人出来是sibling的组合只有60个，而不是60*60。

IShallFly

Prep的答案是3/40000，刚才google了一下，找到了解题方法， 和大家分享

第一种做法：

A = 选2个人出来是sibling有60种组合

B = 任选2个人出来有1000 x 800种组合

所以概率是A/B = 60/(1000 x 800) = 3/40000

第二种做法：

A = 从Junior class的1000人中选出60个的概率是60/1000

B = 然后，要从Senior class的800人中选出和Junior class里面的配对的，概率是60/800 x 1/60

所以A x B = 60/1000 x 60/800 x 1/60 = 3/40000

chris5105077

60/1000*60/800