From my earlier post:
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.
Downtown trains run past Marvin’s stop at, say, 3:00, 3:10, 3:20,…, etc., and uptown trains at 3:01, 3:11, 3:21,…. To go uptown, Marvin must arrive in the 1 minute interval between a downtown and an uptown train.