Answer
$35$ km
Work Step by Step
Distance traveled = (average speed)(time of travel).
Let $s$ be the boat speed on placid water (with no currents).
Sailing against the current, her average speed was $(s-5)$ km/h,
and with the time it took (20 min$=\displaystyle \frac{20}{60}=\frac{1}{3}$hr),
she covered $(s-5)\displaystyle \cdot\frac{1}{3}$ km.
Downstream, her speed was $(s+5)$ km/h, and the time was $15$min$=\displaystyle \frac{1}{4}$h.
The distance she covered was $(s+5)\displaystyle \cdot\frac{1}{4}$ km.
The distances discussed are equal.
$(s-5)\displaystyle \cdot\frac{1}{3}=(s+5)\cdot\frac{1}{4}\qquad$ ... multiply both sides by 12 (LCM of 3 and 4)
$4(s-5)=3(s+5)$
$ 4s-20=3s+15\qquad$ ... add $20-3s$
$s=35$ km
