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60 siblings’ pairs

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by sanju09 » Sat Feb 21, 2009 2:17 am
A certain junior class has 1,000 students and a certain senior class has 800 students. Among these students, there are 60 siblings’ 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 at will be a sibling pair?

A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15
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by sureshbala » Sat Feb 21, 2009 2:42 am
Total number of favorable cases = 1000 x 800

Now in order to have a sibling pair, we have to select the senior student from any one of the 60 pairs available. Once we select the senior, we have to select the junior obviously from the same pair. So the favorable chances will be 60.

Hence required probability = 60/1000x800 = 3/40,000
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by sanju09 » Sat Feb 21, 2009 3:13 am
sureshbala wrote:Total number of favorable cases = 1000 x 800

Now in order to have a sibling pair, we have to select the senior student from any one of the 60 pairs available. Once we select the senior, we have to select the junior obviously from the same pair. So the favorable chances will be 60.

Hence required probability = 60/1000x800 = 3/40,000
:) balle-balle te shawa-shawa :lol:
The mind is everything. What you think you become. -Lord Buddha



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