Question

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

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

Answer 1

Answer: (B)

$3$

Speed of boat, $v_b=5km/h$
Speed of boat when water flows, $v_r=\frac{1}{\frac{1}{4}}=4km/h$
Resultant speed $v=\sqrt{{v_b}^2-{v_r}^2}=\sqrt{5^2-4^2}=\sqrt{25-16}=\sqrt{9}=3km/h$