A man is 4/7 across a train trestle when he sees a train coming. To get off the trestle, he can run toward the train or away from it. As it happens, in either case he’ll reach safety just as the locomotive passes him. If he runs at 20 kph, how fast is the train going?
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If he runs toward the train, he’ll cover the remaining 3/7 of the trestle and meet the train at the far end. This means that if he runs away from the train, he’ll have covered 3/7 of the trestle (with 1/7 to go) when the train reaches the far end. He’ll then run the remaining 1/7 while the locomotive crosses all 7/7 and passes him. So the train is traveling at 7 times his speed, or 140 kph.
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