Watch Out for the Train


A man is walking along a one-way railway bridge.  He is two-thirds of the way over it when he sees a train approaching him at a rate of 45 miles per hour.  In this situation he is able to escape in the nick of time by running at a constant speed.  The interesting thing is that it makes no difference in which direction he runs.  Without any algebra or any equation, can you calculate his speed?



Because the man is two thirds of the way across the bridge, he can run forward one-third or backward one-third. At that moment, the train has just reached the bridge. If the man is able to escape by running backward, it means that the train covers the whole length of the bridge in the time the man covers the remaining third. Therefore, the man runs at one-third the speed of the train, or 15 miles per hour.
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