A boat goes 24 km downstream in 10 hours, it takes 2 hours more to cover the same distance against the stream. What is the speed of the boat in still water?

A. 2km/hr
B. 2.8 km/hr
C. 4 km/hr
D. 4.2 km/hr
E. None of these
Explanation:
Speed downstream = 24/10 km/hr = 2.4 km/hr
Speed upstream = 24/12 km/hr = 2 km/hr
Speed of the boat in still water = ½ (2.4 +2)km/hr = 2.2 km/hr