A swimmer can swim with velocity of 12 km/h in still water. Water flowing in a river has velocity 6 km/h. The direction with respect to the direction of flow of river water is $(112+\theta )$ should swim in order to reach the point on the other bank just opposite to his starting point is (in degree). Find the value of   $\theta$

$V_{MR}=$  velocity of man = 12 km/h and 
$V_R=$  velocity of water flow in river  $=6km/h$
As, $V_{MR}$  should be along CD 

$$\begin{gathered} \Rightarrow V_R-V_{MR}\sin \theta =0\Rightarrow 6-12\sin \theta =0 \\ \Rightarrow \sin \theta =\frac{6}{12}\Rightarrow \sin \theta =\frac{1}{2} \\ \Rightarrow \theta {\sin }^{-1}\left(\frac{1}{2}\right)={\sin }^{-1}\left(\sin {30}^0\right)[\because \sin {30}^0=\frac{1}{2}] \\ \Rightarrow \theta ={30}^0 \therefore \alpha ={90}^0+\theta ={90}^0+{30}^0={120}^0\Rightarrow \alpha ={120}^0 \end{gathered}$$