Question

A boat travels with a speed of $15$ km/hr in still water. if 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 $8:9$

Speed of boat $=15$ km/hr

Speed of river $=5$ km/hr

Let distance traveled $=s$

Average speed $=\frac{distance}{time}=\frac{2s}{\frac{s}{20}+\frac{s}{10}}$

Average speed $=\frac{2s}{\frac{3s}{20}}=\frac{40}{3}$

$\therefore$ required ratio $=\frac{\frac{40}{3}}{15}=\frac{8}{9}$