Question

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

Answer 1

$v=\frac{1}{15/60}=4kmph$

$v_R=?$

$v_B=5kmph$

$\sin \theta =\frac{v}{v_B}=\frac{4}{5}$

$\theta ={53}^o$

$\cos \theta =\frac{v_R}{v_B}$

$v_R=v_B\cos \theta$

$v_R=5\times \cos 53=3kmph$

$v_R=3kmph$

Therefore speed of river is 3 kmph.