Question

A motorboat whose speed in still water is 10 km/h went 91 km downstream and then returned to its starting point. Calculate the speed of the river flow if the round trip took a total of 20 hours.

• A. 3 km/h
• B. 4 km/h
• C. 6 km/h
• D. 8 km/h

Answer 1

The correct option is A 3 km/h

Look for the solution by thinking of the factors of 91. It can be seen that $\frac{91}{13}+\frac{91}{7}=7+13=20$ hours. This means that the speed of the boat in still water is 10 kmph and the speed of the water flow would be 3 kmph. Option (a) is correct.