A swimmer covers 30 km downstream and 18 km upstream, taking 3 hours for each journey. What is the swimmer's speed in still water?

Explanation

The downstream speed is 30 km ÷ 3 hours = 10 km/h, and the upstream speed is 18 km ÷ 3 hours = 6 km/h. The swimmer's speed in still water is the average of these two speeds: (10 + 6) ÷ 2 = 8 km/h.