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A jet flew from Tokyo to Bangkok, a distance of 4800km. On the return trip the speed was decreased by 200km/h. If the difference times of the fights was 2 hours, what was the speed from Bangkok to Tokyo.

  • Math -

    time for 1st flight = 4800/x
    time for return flight = 4800/(x-200)

    4800/(x-200) - 4800/x = 2
    multiply by x(x-200)
    4800x - 4800(x-200) = 2x(x-200)
    960000 = 2x^2 - 400x
    x^2 - 200x - 480000=0
    (x - 800)(x + 600) = 0
    x = 800 or x = -600 , the negative speed makes no sense

    so x = 800 km/h

    check:
    1st flight = 4800/800 = 6 hrs
    2nd flight = 4800/600 = 8 hrs
    difference is 2 hours, YEahh!

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