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    Question

    Two trains running on parallel track with lengths 140 m

    and 40 m respectively. When travelling in opposite directions they are observed to pass each other in 10 seconds, but when they are running in the same direction at the rates as before, the faster train passes the other in 20 seconds. Find the speed of the two trains in km per hour.
    A 32 km/hr and 16 km/hr Correct Answer Incorrect Answer
    B 42.5 km/hr and 15 km/hr Correct Answer Incorrect Answer
    C 53.6 km/hr and 18.2 km/hr Correct Answer Incorrect Answer
    D 48.6 km/hr and 16.2 km/hr Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Let R1 = Relative speed, when moving in same direction = (140 + 40)/20 = 9 m/s R2 = Relative speed, when moving in opposite direction = (140 + 40)/10 = 18 m/s Now, Speed of faster train = (R1+R2)/2 = (9 + 18)/2 = 13.5 m/sec = 13.5 x 18/5 = 48.6 km/hr Speed of slower train = (R2-R1)/2 = (18-9)/2 = 4.5 m/s = 4.5 x 18/5 = 16.2 km/hr

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