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medium probability Q

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medium probability Q

by kath_the_great4 » Thu Jun 05, 2008 5:13 pm
A certain junior class has 1000 students and certain senior class has 800 students. Among these students 60 are sibling pairs 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 students selected will be a sibling pair?

a) 3/40,000
b) 1/3600
c) 9/2000
d) 1/60
e) 1/15

OA is a)

Please help with the steps
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by chidcguy » Thu Jun 05, 2008 7:32 pm
You can pick one senior and one junior in 800 X 1000 ways

You can pick a sibling pair in only 60 ways

P=60/80000 = 3/40K
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by atlantic » Sun Jun 15, 2008 9:04 am
CGuy, I must confess that I couldnt clear undestand your answer.

Let me see, the chance of picking one of the siblings from the junior class is 60/1000, correct? Yes. Then you must pick the sibling of this guy on the junior class. And, of course there is only one possibe choice, i.e. 1/8000.

Hence, 60/1000 * 1/800 = 60/8000 = 3/40,000.
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