Answer
$3$ mph
Work Step by Step
time = $\displaystyle \frac{\text{distance}}{\text{rate}}$
let x = water current's rate
We are given that $\displaystyle \frac{20}{7+x}=\frac{8}{7-x}$
... multiply with the LCD, $(7-x)(7+x)$
$20(7-x)=8 (7+x)$
$ 140-20x=56+8x \qquad$ ... add $20x-56$
$140-56=8x+20x$
$ 84=28x\qquad$ ...$ /\div 28$
$3=x$
