A boat moves at 10 km/h in still water in a stream flowing at 5 km/h. What is its average speed for a trip downstream and back?

Explanation

1. 01

   For a distance $d$ each way: time $= \frac{d}{15} + \frac{d}{5} = \frac{4d}{15}$.

2. 02

   Average speed $= \frac{2d}{4d/15} = 7.5$ km/h, which is $\frac{b^2 - s^2}{b}$.