A man can row a boat at 10 kmph in still water. If the speed of the stream is 6 kmph, the time taken to row a distance of 80 km down the stream is :

A. 8 hours
B. 10 hours
C. 5 hours
D. 20 hours
Answer: C

Explanation:  

Speed downstream (10+6) km/hr 16 km/hr.
Time taken to cover 80 km downstream = (80/16) hrs
                                                             = 5 hrs.