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    Question

    A train can cross a bridge and a pole in 7.2 seconds and 4

    seconds respectively. The length of the bridge is 80 metres. Find the time taken by train to cross a car which is running in the same direction with a speed of 20 m/sec.
    A 22 seconds Correct Answer Incorrect Answer
    B 20 seconds Correct Answer Incorrect Answer
    C 25 seconds Correct Answer Incorrect Answer
    D 18 seconds Correct Answer Incorrect Answer

    Solution

    Let the length and the speed of the train be 'x' metres and 'y' m/sec respectively.
    When it crosses the bridge,
    Or, {(x + 80) / y} = 7.2.................(I)
    When it crosses the pole,
    Or, (x / y) = 4
    So, x = 4y............(II)
    Or, 4y + 80 = 7.2y
    Or, 80 = 3.2y
    So, y = (80 / 3.2) = 25 m/sec
    And x = 25 × 4 = 100 metres
    So, relative speed of the train w.r.t. the car = 25 - 20 = 5 m/sec
    So, required time = (100 / 5) = 20 seconds

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