Question

A boat which has a speed of $5\text{km/h}$ in still water crosses a river of width $1\text{km}$ along the shortest possible path in $15\text{min}$. The velocity of the river water in $\text{km/h}$ is

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

Answer 1

The correct option is B $3$


Shortest path taken $s=1\text{km}$
Time taken $=15\text{min}$
Speed along $AB=\sqrt{v^2-u^2}=\frac{s}{t}$
$\Rightarrow \frac{1}{0.25}=\sqrt{5^2-u^2}\Rightarrow u=3\text{kmph}$