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Stand By Me
John was trying to take a short cut through a very narrow tunnel when he heard the whistle of an approaching train behind him. Having reached three-eighths of the length of the tunnel, he could have turned back and cleared the entrance of the tunnel, running at 10 miles per hour, just as the train entered. Alternatively, if he kept running forward at that same rate of speed, the train would reach him the moment he would jump clear of the tracks. At what speed was the train moving?
Answer
At 40 miles per hour, the train would enter the tunnel when John was still two-eighths from the exit or a quarter of the tunnel's length. If the train was to reach him at the exit, it would have to travel at four times John's speed, i.e. 40 miles per hour.Hide Answer Show Answer
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