A boat moves relative to water with a velocity which is n=2.0 times less than the river flow velocity. At what angle to the stream direction must the boat move to minimize drifting?

When the velocity of river is greater than the velocity of boot or swimmer, then the motion of boat or swimmer without drifting is not possible

${\nu }_r\to$  velocity of the river flow ; $OA=\ell$  width of the river

$\nu \to$  velocity of the boat w.r.t still water and ${\nu }_r=n\nu$

$\theta \to$  angle made by the boat with the river flow.

Fromm figure

Drift (x) $=\left(\nu \cos \theta +{\nu }_r\right)t\to \left(1\right)$

$OA=\ell =\nu \sin \theta t\to \left(2\right)$

From (1) & (2) we get, $x=\left(\cot \theta +\frac{{\nu }_r}{\nu }\sec \theta \right)\ell$  for x to be minimum

with $\theta$

$\frac{dx}{d\theta }=0\Rightarrow \cos \theta =-\frac{1}{n}=-\frac{1}{2}\Rightarrow \theta ={120}^0$