Question

A boat which has a speed of 5 km/h in still water crosses a river of width 1 km along the shortest possible path in 15 minutes. The velocity of the river water in km/h is:

• A. $1$
• B. $3$
• C. $4$
• D. $\sqrt{41}$

Answer 1

The correct option is B $3$


Resultant velocity of boat,
$v=\frac{1km}{15min}=\frac{1km}{\left(\frac{1}{4}\right)h}=4km{h}^{-1}Velocityofriver,v_R=\sqrt{v_B^2-V^2}=\sqrt{5^2-4^2}=3km{h}^{-1}$