请问一道prep 题

sdcar2010

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

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]