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# A certain junior class has 1000 students and a certain

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Intern
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A certain junior class has 1000 students and a certain [#permalink]  05 Jan 2008, 13:23
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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 2 students selected will be a sibling pair?
A: 3/40000
B: 1/3600
C:9/2000
D: 1/60
E:1/15
OA is A.
Can someone please explain this problem in detail?
Logic says since there are 60 sibling pairs:
There are 30 seniors and 30 juniors
So probability is p= 30/1000* 30/800.
Why is this wrong?
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Re: Probability [#permalink]  05 Jan 2008, 13:41
60 pairs of siblings = 120 people; 60 juniors and 60 seniors; Probability of selecting a junior out of the class of 1000 people is 60/1000. Probability of selecting his/her sibling out of the class of seniors is 1/800 (since only one person can be his/her sibling);

So we have 60/1000 * 1/800 = 3/40000
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Re: Probability [#permalink]  05 Jan 2008, 13:46
Oops. I got the question wrong then. Thanks that makes sense.
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Re: Probability [#permalink]  27 Sep 2009, 09:54
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 2 students selected will be a sibling pair?
A: 3/40000
B: 1/3600
C:9/2000
D: 1/60
E:1/15

Soln: Total no. of ways of choosing two people, one from each class is 800 * 1000.
Of these total outcomes, there are only 60 pairs which are sibling pairs. Hence answer is
= 60/(800 * 1000)
= 3/40000

ans is A
Re: Probability   [#permalink] 27 Sep 2009, 09:54
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