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    Question

    Two trains ‘L’ and ‘M’ of lengths 720 metres and

    480 metres respectively are travelling towards each other. Ratio of speed of ‘L’ to ‘M’ is 5:3. If the trains take 60 seconds to cross each other from the time they meet, then find the average of speeds (in km/h) of trains ‘L’ and ‘M’.
    A 18 km/h Correct Answer Incorrect Answer
    B 22 km/h Correct Answer Incorrect Answer
    C 28 km/h Correct Answer Incorrect Answer
    D 36 km/h Correct Answer Incorrect Answer
    E 42 km/h Correct Answer Incorrect Answer

    Solution

    Let the speeds be 5y and 3y m/s respectively.
    Relative speed = 5y + 3y = 8y m/s
    ATQ,
    8y × 60 = 720 + 480
    ⇒ 480y = 1200
    ⇒ y = 2.5
    Therefore, required average = (5y + 3y) ÷ 2 = 4y
    = 4 × 2.5 = 10 m/s = 10 × (18/5) = 36 km/h

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