Please Help-Probability

[cubicle_bound_misfit]

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


Regards,

[sudhir3127]

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.

[cubicle_bound_misfit]

Thanks.
That's what I did my friend.
But the OA is A? :!:

Can someone please explain why?


Regards,

[sudhir3127]

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.