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probability of siblings

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by VivianKerr » Wed Feb 16, 2011 1:31 pm
Junior Class - 1000 students
Senior Class - 800 students

60 pairs (120 total) with 1 junior/1 senior

1 student selected from EACH class - probability that the students will be a sibling pair?

We need TWO things to happen: (1) choose a student who is a sibling pair, (2) choose their sibling as the 2nd person

Let's say we choose a junior first:

60/1000 juniors are in a pair, then the odds are 1/800 we will choose their sibling. Multiply together gives us:

60/1000 * 1/800 = 60/800,000 = 6/80,000 = 3/40,000

The answer is [spoiler](A)[/spoiler].
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by Pilot » Thu Feb 17, 2011 12:14 pm
Is my solution correct?

First i counted all possible pairs, which is equal to 800*1000. Then we have only 60 siblings. That's why I divide 60 to 1000*800. The answer is 60/1000*800=3/40000

Thanks!!!
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