116. 
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?

以下计算思路是否正确:

60/1000*60/800* 1/60=3/40000 (即,从1000和800里被挑出的可能性是第一步,第二步是被挑出来的60个junior和60个senior 彼此配对的可能性为1/60 )

求事件总和：每个年级各来一个1000×800

条件事件总和：60

概率=60/（1000×800）