Question

A boat having a speed of $5km/h$ in still water, crosses a river of width $1km$ along the shortest possible path in $15min$. The speed of the river water in $km/h$ is

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

Answer 1

The correct option is B $3$

The velocity of the boat in still water$=5km/h$
To cross the river in the shortest possible path it has to be done perpendicular to the flow of the river.
Velocity of boat in water moving perpendicular to flow $=1/0.25=4km/h$
Let $v$ be the velocity of the flow of the river.

$5^2=4^2+v^2+2\times 4\times vcos90^o{v}^2=25-16=9v=3km/hr$