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    Question

    A train covers a certain distance at a speed of 12 km/h

    and the next same distance with a speed of 21 km/h and the next same distance with a speed of 36 km/h. find the average speed?
    A 12.6 km/hr Correct Answer Incorrect Answer
    B 30 km/hr Correct Answer Incorrect Answer
    C 45 km/hr Correct Answer Incorrect Answer
    D 18.9 km/hr Correct Answer Incorrect Answer

    Solution

    Since the distance is same in all the conditions. Therefore, Average speed = 3xyz/(xy + yz + zx) Average speed = 3 x 12 x 21 x 36/(12 x 21 + 21 x 36 + 12 x 36) Average speed = 3 x 12 x 21 x 36/(252 + 756 + 432) = 27216/1440 = 18.9 km/hr    

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