Question

A boat travels with a speed of 15 km/hr in still water. In a river flowing at 5 km/hr, the boat travels some distance downstream and then returns. The ratio of average speed to the speed in still water is

• A. 8 : 3
• B. 3 : 8
• C. 8 : 9
• D. 9 : 8

Answer 1

The correct option is C 8 : 9

Time taken in downstream $=\frac{d}{15+5}=\frac{d}{20}$
Time taken in upstream $=\frac{d}{15-5}=\frac{d}{10}$
$\therefore$ Average speed $=\frac{2d}{\frac{d}{20}+\frac{d}{10}}=\frac{40}{3}km/hr$
So, $\frac{\text{Average speed}}{\text{Speed in still water}}=\frac{40/3}{15}=\frac{40}{45}=\frac{8}{9}$.