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The speed of a river current is 4 mph. IF a boat travels 20 miles downstream in the same time that it takes to travel 10 miles upstream, find the speed of the boat in still water.

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  1. downstream speed = u + 4
    upstream speed = u - 4

    20 = (u+4) t
    10 = (u-4)t

    so
    20/(u+4) = 10/(u-4)

    10 u + 40 = 20 u - 80

    10 u = 120
    u = 12 miles/hr

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