In a river, a man takes 3 hours in rowing 3 km upstream or 15 km downstream. What is the speed of the current?

A. 2km/hr
B. 4km/hr
C. 6km/hr
D. 9km/hr
Explanation:
Speed upstream = 3/3 km/hr =1 km/hr
Speed downstream = 15/3 km/hr = 5 km/hr
Speed of current = ½(5 -1) km/hr = 2 km/hr