Question 1107789
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Jane took 30 min to drive her boat upstream to water-ski at her favorite spot. 
Coming back later in the& day, at the same boat speed, took her 15 min. 
If the current in that part of the river is 6 km per hr, what was her boat speed in still water?
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Let boat speed in still water be x
The speed of the current = 6 km/h

upstream speed = (x-6) km/h.
downstream speed = (x+6) km/h


distance = time * speed


Distance upstream is  1/2 of an hour * (x-6) km/h

    d = {{{(1/2)*(x-6)}}} kilometers.


Distance downstream is  1/4 of an hour * (x+6) km/h

    d = {{{(1/4)*(x+6)}}} kilometers.


The distance is the same in both directions - - - so, we have this equation 

    {{{(1/2)*(x-6)}}} = {{{(1/4)*(x+6)}}}     (1)


Multiply equation (1) by 4  (both sides).  We get

    2(x-6) = x + 6,

    2x - 12 = x + 6,

    2x - x = 12 + 6,

       x   =    18.


<U>ANSWER</U>.  The speed of boat in still water is 18 km/h.
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