A man rows 30 km downstream and 20 km upstream in a total of 4 hours. He also rows 45 km downstream and 40 km upstream in 7 hours. What is his speed in still water?

Explanation

Let the man's speed in still water be 'a' km/h and the speed of the stream be 'b' km/h. From the given information, we have two equations: 30/(a + b) + 20/(a - b) = 4 and 45/(a + b) + 40/(a - b) = 7. Solving these equations simultaneously, we find that the speed of the man in still water is 12.5 km/h.