A person can row 7 1/ 2 km/h in still water. It takes him twice as long to row up a distance as to row down the same distance. Find the speed of the stream?

Speed upstream + Speed downstream = 2 x 7¹/₂ = 15 km/h
Since, the time taken is in the ratio 2 : 1
hence the speed will be in the ratio 1 : 2
∴ Speed upstream = (1/3) x 15 = 5 km/h
and speed downstream = (2/3) x 15 = 10 km/h
∴ Speed of stream = (10 - 5)/2 = 2.5 km/h