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# A certain class has 1000 students and a certain class has

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Manager
Joined: 22 May 2007
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A certain class has 1000 students and a certain class has [#permalink]

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29 May 2008, 22:02
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

A certain class has 1000 students and a certain class has 800 students. among these students, there are 60 sibling pairs, each consisting of 1 junior and 1 senior. If a student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?

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Director
Joined: 23 Sep 2007
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Re: Probability question - Need help [#permalink]

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29 May 2008, 22:14
if you pick 1 student out of 1000 seniors, your chance of picking 1 sibling is 60/1000

your odd of a second pick of a matching sibling is 1/800

multiply them together = 60/80000 = 3/40000
Manager
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Re: Probability question - Need help [#permalink]

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29 May 2008, 23:32
y can it not be 60/800 ?
Manager
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Re: Probability question - Need help [#permalink]

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30 May 2008, 04:03
aaron22197 wrote:
y can it not be 60/800 ?

It cannot be 60/800 because we dont have to pick 'any one' of the 60 sibling pairs present.
We have to pick THAT ONE person whose sibling we have already selected( 60/1000).
This is conditional probability .
Hope that helps.

Cheers !
Re: Probability question - Need help   [#permalink] 30 May 2008, 04:03
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