求助一道prep题~（我数学差，莫鄙视）

devilmona

题目大意是：有A班1000人，B班800人，现在要抽60组人，每一组人由一个A班的人和一个B班的人组成，问在A班的甲和B班的乙被抽到一组的概率？？？概率完全忘光了~~~答案是3/4000(貌似没记错)
求牛人指点~~

devilmona

求助啊？还是被鄙视了？

uwants

60/(1000*800)

uwants

A certain junior class has 1000 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 2 students selected will be sibling pair
1) 3/40,000
2)1/3,600
3)9/2,000
4)1/60
5)1/15
Assume the junior siblings in the 60 sibling pairs are numbered as Js(junior sibling)1, Js2, Js3, .... , Js60. And there are 1000 students in a junior class.
Also assume that the senior siblings in the 60 sibling pairs are numbered as Ss(senior sibling)1, Ss2, Ss3, .... , Ss60. And there are 800 students in a senior class.

We get to pick one from each class, a junior class and a senior class, and have to find the probability that the two will be a pair, i.e. (Js1, Ss1), (Js2, Ss2), ... , (Js60, Ss60).

Solution:
1. There are 60 possible siblings in a junior class to choose from = 60/1000 (here we do not care which sibling we pick as long as we pick the exact match in a senior class)

(Two different interpretations are possible from here)
2a. There are 60 siblings in a senior class and we must match our pick out of 60 the one we picked from the junior class = (60/800)*(1/60); or
2b. We do not care how many siblings in a senior class because we just have to pick the match out of 800 seniors = 1/800

1 * 2a = (60/1000)*(60/800)*(1/60) = 3/40000
1 * 2b = (60/1000)*(1/800) = 3/40000