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It takes mary x hours to travel 100 kilometers. If she increases her speed by 5 kilometers per hour, the journey will take her 3 hours less time. Find x.

  • math -

    her original speed be v
    then vx = 100
    her faster speed = v+5
    (x-3)(v+5) = 100
    xv + 5x - 3v - 15 = 100
    100 + 5x - 3v = 115
    5x - 3v = 15
    v = (5x-15)/3

    sub into 1st
    x((5x-15)/3) = 100
    5x^2 -15x = 300
    x^2 - 3x - 60 = 0
    I get x = 9.39 by using the formula (and v = 10.65)

    check: (9.39)(10.65) = 100.0035
    (6.39)(15.65) = 100.0035
    YEAHH!

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