Mario drew a model to represent one third of the space in his bookcase. how could mario draw a model to represent all the space
2 answers:
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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.
