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A certain junior class has 1000 students and a certain

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A certain junior class has 1000 students and a certain [#permalink] New post 14 Apr 2006, 18:36
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?
- 3/40,000
- 1/3600
- 9/2000
- 1/60
- 1/15
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Re: Prob Q (GMATPrep) [#permalink] New post 14 Apr 2006, 18:52
ashkapoo wrote:
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?
- 3/40,000
- 1/3600
- 9/2000
- 1/60
- 1/15


p (sibling) = (60/1,000) (1/800) = 3/40,000
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 [#permalink] New post 14 Apr 2006, 19:00
Would'nt it be (60/1,000)*(1/800) + (60/800)*(1/1,000), depending on whether you pick a senior or junior first?
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 [#permalink] New post 14 Apr 2006, 19:07
ashkapoo wrote:
Would'nt it be (60/1,000)*(1/800) + (60/800)*(1/1,000), depending on whether you pick a senior or junior first?


because thre are equal junier and sinior siblings in each class.

(60/1,000)*(1/800) or (60/800)*(1/1,000) are same.

you have 60 chances out of either 1000 or 800 to get a sibling from either sinior or junior. then you go to next, which could be either...
  [#permalink] 14 Apr 2006, 19:07
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