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Bus wait when some gaps include an hour-long refuel

Question

Buses are scheduled 1010 minutes apart. Before each gap, independently, the driver stops to refuel with probability 10%10\%, and a refuel adds 6060 minutes to that gap (making it 7070 minutes). You arrive at the stop at a uniformly random time of day. What is your expected wait, in minutes, for the next bus?

Short answer

1458\tfrac{145}{8} minutes (18.125). You are more likely to land in a long gap (inspection paradox). Your expected wait is E[L2]2 E[L]\tfrac{\mathbb{E}[L^2]}{2\,\mathbb{E}[L]} for gap length LL: E[L]=16\mathbb{E}[L]=16 and E[L2]=0.9⋅100+0.1⋅4900=580\mathbb{E}[L^2]=0.9\cdot100+0.1\cdot4900=580, giving 58032=1458\tfrac{580}{32}=\tfrac{145}{8}.

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

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