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)

L= Length of the tunnel
S= Speed of the Train

(1/4)*(L)*(speed of man) = (3/4)*(L)*(speed of man) + (L)*(speed of Train)

after simplifying and substituting for the speed of the man:

(speed of Train) = 1 mph

Posted by Andrew on 2002-05-26 16:04:42