If a passenger sitting near the window in a train moving at 40 kmh noticed that it took 3 seconds for the oncoming train to pass by, what was the speed of the oncoming train if the length of the oncoming train was 75 meters.
Think of the passenger as a moving point with a speed of 40 kmph. The oncoming train took 3 secs to pass by this passenger which means that 75 m were covered by the passenger and the oncoming train in a total of 3 secs. So total speed = 75/3 = 25 m/s
Converting it to kmph, we get 25 * (18/5) = 90 kmph
So together the passenger and the oncoming train are moving at a speed of 90 kmph with respect to each other. Since the speed of the passenger is 40 kmph, the speed of the oncoming train must be 50 kmph.
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