Question 1117143
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Let x be the rate on the first river.


Your "time" equation is


{{{18/x}}} + {{{10/(x-4)}}} = 8   hours.


It is "time" equation, because each fraction in the left side represents the time traveled on corresponding river.


To solve it, multiply both sides by x*(x-4). You will get a quadratic equation.
Simplify it and solve it by any method you know/(you like).


The answer is x= 6 kilometers per hour.  (I solved it mentally).
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