A boat moves at 12 km/h in still water in a stream flowing at 4 km/h. What is its average speed for a trip downstream and back over the same distance?

Explanation

1. 01

   Downstream 16 km/h, upstream 8 km/h. Over equal distances the average speed is $\frac{2 \times 16 \times 8}{16 + 8}$.

2. 02

   $= \frac{256}{24} = \frac{32}{3}$ km/h, about 10.7 km/h.