Triangle ABC is congruent to triangle DFE. Find y.
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The answer is D.
The answer relies on whether the balls are different or not. If they are not, which is almost certainly what is intended.
If they are, the perceptive is a bit different. Your expression gives the likelihood that a particular set of j balls goes into the last urn and the other n−j balls into the other urns. But there are (nj) different possible sets of j balls, and each of them the same probability of being the last insides of the last urn, so the total probability of completing up with exactly j balls in the last urn is if the balls are different.
See attached file for the answer.
