A swimmer goes downstream at 7.5 km/h and upstream at 4.5 km/h. What is the speed in still water?

Explanation

1. 01

   Downstream $= b + s$ and upstream $= b - s$.

2. 02

   $b$ is their average: $\frac{\frac{15}{2} + \frac{9}{2}}{2} = 6$ km/h.