Vector Algebra

A boat crosses a river from one bank A to another bank B which is opposite. The distance between them is D. The speed of water is $V_W$ and that of boat relative to water is $V_B$. If $V_B = 2V_W$, the time taken by the boat to cross the river directly along AB is $\left(\sin 30^\circ = \dfrac{1}{2}\right)$, $\left(\cos 30^\circ = \dfrac{\sqrt{3}}{2}\right)$