A swimmer can move at a speed of 4 km/h in still water. If the river current flows at 2 km/h, how long will it take him to swim 6 km upstream against the current?

Explanation

The swimmer's speed in still water is 4 km/h, and the current speed is 2 km/h. When swimming against the current, his effective speed is 4 - 2 = 2 km/h. To cover 6 km upstream, time taken = distance / effective speed = 6 / 2 = 3 hours.