A man rows at a speed of 9 1/3 km/hr in still water. He notices that rowing upstream takes three times as long as rowing downstream over the same distance. What is the velocity of the river current?

Explanation

Let the upstream speed be x km/hr and the downstream speed be 3x km/hr since upstream time is thrice downstream time for the same distance. The average speed in still water is (x + 3x) / 2 = 2x km/hr. Given that the speed in still water is 9 1/3 km/hr (which is 28/3 km/hr), we have 2x = 28/3, so x = 14/3 = 4 2/3 km/hr. Therefore, the speed of the current is 4 2/3 km/hr.