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Poisson arrivals spaced over a minute until a woman

Question

Customers arrive at a cinema as a Poisson process with rate one per minute. Independently, each arrival is a man with probability 23\tfrac23 and a woman otherwise. Let NN count the arrivals up to and including the first woman. What is the probability that each of the first NN interarrival times is longer than one minute? Writing the answer as abe−c\dfrac{a}{be-c} with a,b,ca,b,c coprime integers, give abcabc.

Short answer

6. Here NN is geometric with P(N=n)=2n−13nP(N=n)=\tfrac{2^{n-1}}{3^n}, independent of the Exp(1)\text{Exp}(1) gaps, each of which exceeds 1 with probability e−1e^{-1}. Summing ∑n2n−13ne−n\sum_n \tfrac{2^{n-1}}{3^n}e^{-n} as a geometric series gives 13e−2\tfrac{1}{3e-2}, so abc=1⋅3⋅2=6abc=1\cdot3\cdot2=6.

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