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a boy walks to a school at a rate of 6km with a speed of 2.5km/h and walks back with a constant speed of 5km/h. his average speed for round trip in km/h is ?

  • physics -

    You must mean that the rate of walking to school is 6 km/h, not 6 km.

    Let the distance to school be D. (It will cancel out when you compute the avg. speed)

    avg speed = (total distance covered)/(total time)
    = 2D/[D/6 + D/2.5}
    = 2/[(1/6 + 4/10]
    = 2/[5/30 + 12/30]
    = 60/17 = 3.53 km/h

    Note that this is NOT the average of 6 and 2.5. He spends more time walking at the lower speed.

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