A man swims 72 km downstream and 45 km upstream, taking 9 hours for each journey. What is the velocity of the current?

Explanation

The downstream speed is 72 km ÷ 9 hours = 8 km/h, and the upstream speed is 45 km ÷ 9 hours = 5 km/h. Let the swimmer's speed in still water be S and the current's speed be C. Then, S + C = 8 and S - C = 5. Adding these equations gives 2S = 13, so S = 6.5 km/h. Subtracting gives 2C = 3, so C = 1.5 km/h. Therefore, the speed of the current is 1.5 kilometers per hour.