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you go up a hill at 10 miles per hour and down the hill at 30 miles per hour what is the average speed and it's not 20 miles per hour

  • math -

    time to go up hill = d/10
    time to go down = d/30
    total time = d/10 + d/30 = 4d/30

    avg speed = total distance / total time
    = 2d/(4d/30)
    = 2d(30/4d)
    = 60/4 = 15 mph

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