请问一道prep 题

dreamstart

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 the 2students selected will be a sibling pair？

sdcar2010

Hard, "complete" way:

When choosing 2 students, there are basically 4 types of scenarios:

1) Choose a "non-sibling" student among the 1000 juniors, then definitely you won't have a sibling pair. Probability: (1000-60)/1000
2) Choose a "sibling" student among the 1000 juniors and a "non-sibling" student from the seniors, then definitely you won't have a sibling pair. Probability: [60/1000] * [(800-60)/800]
3) Choose a "sibling" student among the 1000 juniors and any "sibling" student from the seniors who are not the sibling of the junior choice. Probability: [60/1000] *[59/800]
4) Choose a "sibling" student among the 1000 juniors and the matching "sibling" student from the seniors. Probability: [60/1000]*[1/800]

sdcar2010

Quick and dirty way:

total possibilities of choosing a sibling pair: 60
total possibilities of choosing a pair of students: 1000 * 800

Probability of choosing a sibling pari: 60/(1000*800)

fyhanna

其实是C(1 60)/(C(1 1000)*C(1 800)) 分子是从60对中选一对

-- by 会员 wxch (2011/5/1 9:43:14)

没错啊，算式就是楼上再楼上的嘛

wxch

其实是C(1 60)/(C(1 1000)*C(1 800)) 分子是从60对中选一对

fyhanna

不知道对不对啊  60/(1000*800)

-- by 会员 wxch (2011/5/1 9:00:47)

我认为是这个···二楼的那个是怎么算的，来讲讲

wxch

不知道对不对啊  60/(1000*800)

Emmi

ask for more explaination to the math sentence up-floor...thanks~

sdcar2010

1-[(1000-60)/1000][(800-60)/800]