A boat travels at 25 km/h against the current and 35 km/h with the current. What is the speed of the river's current?

Explanation

The boat's speed upstream (against the current) is 25 km/h, and downstream (with the current) is 35 km/h. The current's speed is half the difference between these two speeds: (35 - 25) / 2 = 5 km/h.