A man rows his boat in a river and takes 3 hours to cover 3 km against the current or 15 km with the current. What is the velocity of the river's current?

Explanation

The speed against the current is 3 km divided by 3 hours, which equals 1 km/h. The speed with the current is 15 km divided by 3 hours, which equals 5 km/h. The current's speed is half the difference between downstream and upstream speeds: (5 - 1) / 2 = 2 km/h.