A young man lives in Manhattan near a subway express station. He is dating two women: one in Brooklyn; one in the Bronx. To visit the woman in Brooklyn he takes a train on the downtown side of the platform; to visit the woman in the Bronx he takes a train on the uptown side of the same platform. Since he likes both women equally well, he simply takes the first train that comes along. In this way, he lets chance determine whether he rides to the Bronx or to Brooklyn. The young man reaches the subway platform at a random moment each Saturday afternoon. Brooklyn and Bronx trains arrive at the station equally often—every 10 minutes. Yet for some obscure reason he finds himself spending most of his time with the woman in Brooklyn: in fact, on the average, he goes there nine times out of 10. Can you decide why the odds so heavily favor Brooklyn?

• I'd suggest you to solve it by yourself before looking at the solution. It's not very hard.