Answer
$2$ mph
Work Step by Step
time = $\displaystyle \frac{\text{distance}}{\text{rate}}$
let x = water current's rate
We are given that $\displaystyle \frac{30}{8+x}=\frac{18}{8-x}$
... multiply with the LCD, $(8-x)(8+x)$
$30(8-x)=18 (8+x)$
$ 240-30x=144+18x \qquad$ ... add $30x-144$
$240-144=18x+30x$
$ 96=48x\qquad$ ...$ /\div 48$
$2=x$
water current's rate = $2$ mph
