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# A certain junior class has 1000 students and a certain

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Manager
Joined: 29 Apr 2006
Posts: 85

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A certain junior class has 1000 students and a certain [#permalink]

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20 May 2006, 20:13
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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

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Director
Joined: 16 Aug 2005
Posts: 937

Kudos [?]: 28 [0], given: 0

Location: France

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20 May 2006, 20:25
total # of pairs=1000x800
sibling pairs=60

P = 60/800000 = 3/40,000

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Manager
Joined: 04 May 2006
Posts: 52

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21 May 2006, 04:25
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

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Senior Manager
Joined: 09 Mar 2006
Posts: 444

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21 May 2006, 04:36
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.

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Manager
Joined: 31 Mar 2006
Posts: 162

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26 May 2006, 04:36
Why are total number of pairs 1000*800 and not 1000+800?

Regards,

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Director
Joined: 10 Oct 2005
Posts: 524

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Location: US

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26 May 2006, 07:55
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

Kudos [?]: 69 [0], given: 0

26 May 2006, 07:55
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