A swimmer can swim at a speed of 4.5 km/h in still water. If it takes him twice as long to swim upstream compared to downstream, what is the velocity of the current?

Explanation

Let the swimmer's speed in still water be 4.5 km/h and the stream speed be x km/h. The downstream speed is (4.5 + x) km/h, and the upstream speed is (4.5 - x) km/h. Given that the time taken upstream is twice the time taken downstream, the relationship between speeds is: downstream speed = 2 × upstream speed. So, (4.5 + x) = 2(4.5 - x). Solving this equation gives x = 1.5 km/h, which is the speed of the stream.