# It took 3 hours to row a boat 18 km against the current. The return trip with the current took 1 1/2 hours. How do you find the speed of the rowboat in still water?

#### Answer:

The speed is 9 km/h.

#### Explanation:

Boat speed = Vb
River speed = Vr

If it took 3 hours to cover 18km, the average speed $= \frac{18}{3} = 6$ km/h

For the return trip, the average speed is $= \frac{18}{1.5} = 12$ km/h

$\left\{\begin{matrix}V b - V r = 6 \\ V b + V r = 12\end{matrix}\right.$

According to the second equation,

$V r = 12 - V b$

Substituting in the first equation:

Vb-(12-Vb)=6)

$V b - 12 + V b = 6$

$2 V b = 6 + 12$

$V b = \frac{18}{2} = 9$