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math

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You jog at 6.6 mi/h for 5.0 mi, then you jump into a car and drive for another 5.0 mi. With what average speed must you drive if your average speed for the entire 10.0 miles is to be 10.4 mi/h?

answer is 24.5 mi/h.


WHY??

  • math -

    let the speed in the car be x mph

    so 5/6.6 + 5/x = 10/10.4
    5/x = 10/10.4 - 5/6.6
    5/x = .20396
    x = 5/.20396
    x = 24.5

    he should drive the last 5 miles at 24.5 mph

    check
    if avg speed is 10.4 mph, then it took
    10/10.4 hours or .9615 hours

    time jogging = 5/6.6 = .7576 hours
    time in car = 5/24.5 = .20408 hours
    for a total time of .96166 hours

    allowing for roundoff errors, close enough!

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