A man was walking through a tunnel. He is 1/4 of the way through when he hears a train approaching the tunnel from behind him.

If he turns and runs back, he will make it out of the tunnel just as the train is entering it (and will save himself by a hair). If he goes forward to the far end of the tunnel, he will also just barely make it, emerging from the tunnel just as the train is about to catch up to him.

If the man's running speed is 7 miles per hour, how fast is the train moving?

(from techInterview.org)

If he runs 1/4 the length of the tunnel, back to the start, the train meets him there, so if he runs the opposite direction for 1/4 the tunnel's length, now he's half way through when the train gets to the start. Since the train can make it to the end of the tunnel by the time he can, and he's already half way through, the train has to be going twice as fast as he is, which is 14mph.

Posted by Carey on 2002-05-27 23:13:20