Question

A boat which has a speed of $13\text{km/h}$ in still water crosses a river of width $1\text{km}$ along the shortest possible path in $12\text{minutes}$. The velocity of the river water in $\text{km/h}$ is

• A. $12$
• B. $10$
• C. $8$
• D. $6$

Answer 1

The correct option is A $12$

$\text{Speed of boat relative to ground}=\frac{1}{\left(\frac{12}{60}\right)}\text{km/hr}=5\text{km/hr}$

$\Rightarrow 5={13}^2-x^2\text{where},x\text{is speed of water}$
$\Rightarrow x=12\text{km/hr}$