A boat can go $90 \mathrm{mph}$ in still water. It takes as long to go 630 miles upstream as it does to go downstream 720 miles. How fast is the current?
A) $1 \mathrm{mph}$
B) $9 \mathrm{mph}$
C) $5 \mathrm{mph}$
D) $6 \mathrm{mph}$

Question

Quizplus · Accepted Answer

The speed of the current can be found by setting the time to travel upstream equal to the time to travel downstream and solving for the current's speed. Let $c$ be the current's speed. The boat's effective speed upstream is $90 - c$ and downstream is $90 + c$. Since time = distance/speed, we have $\frac{630}{90-c} = \frac{720}{90+c}$. Solving this equation for $c$ gives $c = 6 \mathrm{mph}$.

A Boat Can Go 90mph90 \mathrm{mph}90mph In Still Water 1mph1 \mathrm{mph}1mph B) 9mph9 \mathrm{mph}9mph

A Boat Can Go $90 \mathrm{mph}$ In Still Water $1 \mathrm{mph}$ B) $9 \mathrm{mph}$