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July 3, 2015

Homework Help: PROBABILITY

Posted by Anonymous on Wednesday, February 19, 2014 at 2:08pm.

A group of 90 students is to be split at random into 3 classes of equal size. All partitions are equally likely. Joe and Jane are members of the 90-student group. Find the probability that Joe and Jane end up in the same class.

My idea:

the total combination: 90C30
since 2 have to be together then I only have 88 students to arrange in groups fo 30-30-28, and It'll be:

88!/(30! 30! 28!)

But then, should I multiply by 3 or how should I add the two that must remain together.

P = [(??) * 88!/(30! 30! 28!)] / 90C30

Or maybe I'm doing this wrong.

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