A man rows a boat at 4.5 km/h in still water. He travels upstream against a river current flowing at 1.5 km/h to a certain point and then returns downstream to his starting position. What is his average speed for the entire trip?

Explanation

The man's speed in still water is 4.5 km/h and the river current speed is 1.5 km/h. His upstream speed is (4.5 - 1.5) = 3 km/h, and his downstream speed is (4.5 + 1.5) = 6 km/h. The average speed for the round trip is given by the formula: (2 × upstream speed × downstream speed) / (upstream speed + downstream speed) = (2 × 3 × 6) / (3 + 6) = 36 / 9 = 4 km/h.