a boat moves relative to water with a velocity which is n = 2 times less than the river flow velocity. at what angle to the stream direction must the boat move to minimize drifting?

Suppose the angle is $\theta$. Now there is no drift along the direction of the flow of the water, the resultant velocity will be perpendicular to the flow of water. Now the the angle of the resultant is give be
$$\begin{gathered} \tan \varphi =\frac{b\sin \theta }{a+b\cos \theta } \\ \Rightarrow \infty =\frac{2x\sin \theta }{x+2x\cos \theta } \\ \Rightarrow x+2x\cos \theta =0 \\ \Rightarrow \cos \theta =-\frac{1}{2} \\ \Rightarrow \theta =120° \end{gathered}$$