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)

14 mph...if the train will get to the entrance of the tunnel by the time he runs back, it will also be there by the time he reaches the halfway point of the tunnel. Both the train and the man will get to the end of the tunnel at the same time. the formula for speed is distance * time. time is negligable since it's the same; therefore, the train has to cover double the distance in the same amount of time. the only way that would happen is if it is going twice as fast...14 mph.

Posted by orlando on 2002-05-28 07:35:32