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Gift baskets with a random number of item types

Question

A bakery manager picks kk uniformly at random from {1,2,3,4}\{1,2,3,4\}. He then puts together a gift basket of kk items, choosing from kk distinct dessert types; repeats are allowed and only how many of each type matters. What is the expected number of different baskets he could make?

Short answer

49/449/4. For a given kk, baskets correspond to non-negative solutions of x1+⋯+xk=kx_1+\dots+x_k=k, of which there are (2k−1k−1)\binom{2k-1}{k-1} by stars and bars: 1,3,10,351,3,10,35. Averaging, (1+3+10+35)/4=494(1+3+10+35)/4=\tfrac{49}{4}.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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