DAY 274

TWO GIRLFRIENDS

Saul I. Have lives in Manhattan and has two girlfriends whom he likes equally well. One lives in the Bronx, the other in Brooklyn. To call on either one, he has to take the subway from a station near his home. He lets chance determine which girl he will visit each time. If the southbound train arrives first, he takes that and goes to Brooklyn. If the northbound train arrives first, he takes that and goes to the Bronx. Both trains arrive at that subway station every fifteen minutes. Saul reaches the platform at a random time every Sunday afternoon. Why does he find himself visiting the Bronx girl four times out of five?

Answer:

Although the Bronx and Brooklyn trains arrive equally often (every fifteen minutes), The Brooklyn train always arrives three minutes before the Bronx train. The Brooklyn train will come first if Saul arrives during that three-minute interval.

He arrives at random times. Therefore the chances are four out of five that he will take the Bronx train.