Debbie traveled by boat 5 miles upstream to fish in her favorite spot. Because of the 4-mph current, it took her 20 minutes longer to get there than to return. How fast will her boat go in still water?
Let the speed in still water be $\displaystyle x$ mph
When it travels upstream its speed= $\displaystyle x-4$ mph
Time taken to go upstream= $\displaystyle \frac{5}{x-4}$ h
Speed in downstream = $\displaystyle x+4$ mph
Time taken to go downstream or time taken to return back= $\displaystyle \frac{5}{x+4}$ h
And 20 min $\displaystyle = \frac{20}{60}=\frac{1}{3}$ hrs
But according to question...
$\displaystyle \frac{5}{x-4} -\frac{5}{x+4}=\frac{1}{3}$
Solve it and you will get the answer