prep ps1-116

jasonlover

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?
这道题应该怎么做呢？
PRER上木有 答案

haileywang

1000里有60SP的，800里有60SP的，总又有60个组合，C(1 60)\C(1 1000)*C(1 60)\C(1 800)*60=27\100  不知道对吗

cckjenius

In total, there are 1000 X 800 = 800000 combinations.
and there are only 60 combinations which are sibling paris.
So the probability is 60/800000