A boat covers a distance of 30 km in 2 ½ hours running downstream while returning it covers the same distance in 3 ¾ hours. What is the speed of the boat in still water?

A. 8km/hr
B. 12km/hr
C. 14 km/hr
D. 15 km/hr
E. None of these
Explanation:
Speed downstream = (30 x 2/5)km/hr = 12km/hr
Speed upstream = (30 x 4/15) km/hr = 8 km/hr
Speed of boat in still water = ½ (12 + 8)km/hr = 10 km/hr