math algebra

A train averaged 80 km/h for the first half of its trip. How fast must it travel for the second half of the trip in order to average 96 km/h for the whole trip?

The answer is 120 km/ h but how do i set it up and get there? thanks for you help

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  1. say it goes distance d in the first half (I assume half the distance, not half the time - poorly written question)

    2 d = total distance = 96 T
    t = time first half + time second half
    T = t1 + t2
    d = 80 t1
    d = x t2
    so
    2d = 96 (t1+t2)
    2d = 96 (d/80 + d/x)
    2/96 = 1/80 +1/x
    .02083 = .0125 + 1/x
    1/x = .00833
    I get x = 120

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