A boatman goes 2 km against the current of the stream in 1 hour and goes 1 km along the current in 10 minutes. How long will it take to go 5 km in stationary water?

A. 30 minutes
B. 45 minutes
C. 1 hour
D. 1 hour 15 min
Ans: D

Explanation:

Rate downstream = [16/2] kmph = 8 kmph; Rate upstream = [16/4] kmph = 4 kmph.
=> Speed in still water = 1/2(8+4) kmph
                                  = 6 kmph.