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A certain junior class has 1000 students and a certain

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Manager
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A certain junior class has 1000 students and a certain [#permalink] New post 20 May 2006, 19:13
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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 jr and 1 sr. 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) 3/40,000
b) 1/3600
c) 9/2000
d) 1/60
e) 1/15
Director
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 [#permalink] New post 20 May 2006, 19:25
total # of pairs=1000x800
sibling pairs=60

P = 60/800000 = 3/40,000
Manager
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 [#permalink] New post 21 May 2006, 03:25
Can't we look at it this way?

P(A and B) = P(A) * P(B)
--> (60/1000) * (60/800)
--> 9/2000 and hence answer (C) ?

Please explain why not if this is not correct...thnaks
Senior Manager
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 [#permalink] New post 21 May 2006, 03:36
Riya05 wrote:
Can't we look at it this way?

P(A and B) = P(A) * P(B)
--> (60/1000) * (60/800)
--> 9/2000 and hence answer (C) ?

Please explain why not if this is not correct...thnaks


This is incorrect, since you have also included all pairs that
consist of two students who are not members of the same sibling pair.
That's it, both have siblings but they are not siblings of each other.
Manager
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 [#permalink] New post 26 May 2006, 03:36
Why are total number of pairs 1000*800 and not 1000+800?

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Director
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 [#permalink] New post 26 May 2006, 06:55
because 1 person can be selected from sr group in 1000 ways and 1 person can be selected from jr group 800. Now if we want to form a group
possibilities will be 800 * 1000
  [#permalink] 26 May 2006, 06:55
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