PREP 数学PS 求教 （PREP 1 中的136题）

CC梦洁

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?


答案是3/40000

garlicwu

这题好像没那么复杂

所有sibling pair的可能是 60，给了，所有可能的组合是 C(1000, 1)*C(800, 1)，两者一比不刚好 3/40000 吗

CC梦洁

太感谢了。做的时候就是绕不过来。