A steamboat travels downstream from one harbor to another in 4 hours and returns upstream over the same distance in 5 hours. If the current's speed is 2 km/h, what is the distance between the two harbors?

Explanation

Let the distance between the two harbors be x kilometers. The downstream speed is x/4 km/h, and the upstream speed is x/5 km/h. The speed of the current is half the difference between downstream and upstream speeds, so ½(x/4 - x/5) = 2. Simplifying, x/4 - x/5 = 4, which leads to x = 80. Therefore, the distance between the two harbors is 80 kilometers.