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by cubicle_bound_misfit » Sat Jul 12, 2008 9:09 pm
Hi Please help in this question,

A certain junior class has 1000 students and a certain senior class has 800 students. Among these students 60 sibling pairs, each consisting of 1 junior and 1 senior. If 1 student to be selected 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


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by sudhir3127 » Sun Jul 13, 2008 3:30 am
i think the answer is 9/2000.

here it goes..

60 siblings means there are 120 students. therefore..

probability of choosing 1 senior out of 1000 students who is a sibling is

60C1/1000C1.....illy

junior who is a sibling is also 60C1/1000C1

therefore the probability is 60/1000 * 60/800

which is 9/2000.
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by cubicle_bound_misfit » Sun Jul 13, 2008 7:31 am
Thanks.
That's what I did my friend.
But the OA is A? :!:

Can someone please explain why?


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by sudhir3127 » Sun Jul 13, 2008 8:24 am
i think then the only way it can be solved is ,,,,

x = prob of choosing a senior who has a sibling
y = prob of choosing senior's sibling from junior class
Z = prob of having chosen a sibling pair

Z = x.y
= 60/800 . 1/1000
= 3/40000

seems very simple to be correct.
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