Question 1127374
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<pre>

Let "x" be the speed of the boat in still water, in kilometers per hour.


Then its speed relative the bank of the river is x+5 km/h downstream and (x-5) km/h upstream.


The time equation is


    {{{30/(x-5)}}} = {{{40/(x+5)}}}.


    30*(x+5) = 40*(x-5)

    30x + 150 = 40x - 200

    150 + 200 = 40x - 30x

    10x = 350  ====>  x = 350/10 = 35.


<U>Answer</U>.  The boat speed in still water is  35 km/h.
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Solved.