math

A motorboat covers the distance from one pier to the other pier in 4 hours and the way back in 5 hours. What is the speed of the boat in still water, if it covers 70 km with the current in 3.5 hours?

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  1. speed of boat in still water ---- x km/h
    speed of current ------ y km/h

    distance with the current = 4(x+y)
    distance against the current = 5(x-y)

    we know that 3.5(x+y) = 70
    x+y = 20

    but 4(x+y) = 4(20) = 80
    then 5(x-y) = 80
    x-y = 16

    x+y=20
    x-y=16
    add them
    2x = 36
    x = 18 , then y = 2

    state the conclusion

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  2. I didn't ask

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