The Unfair Subway

More probability from Frederick Mosteller:

Marvin gets off work at random times between 3 and 5 P.M. His mother lives uptown, his girlfriend downtown. He takes the first subway that comes in either direction and eats dinner with the one he is first delivered to. His mother complains that he never comes to see her, but he says she has a 50-50 chance. He has had dinner with her twice in the last 20 working days. Explain.

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3 thoughts on “The Unfair Subway

  1. I slept on it and when I woke up the answer came to me in the form of a dream. hahaha i’ll post the answer in a few days time – meanwhile let’s wait for replies!

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