Hint: note that $\widehat{CAD} = 15^\circ $ and $\widehat{ABD} + \widehat{BAD} = 135^\circ$. Apply the sine law twice to get:
$$\frac{AD}{CD} = \frac{\sin 30^\circ}{\sin 15^\circ} = \frac{\sin(135^\circ -\widehat{BAD})}{\sin \widehat{BAD}} $$
Hint: note that $\widehat{CAD} = 15^\circ $ and $\widehat{ABD} + \widehat{BAD} = 135^\circ$. Apply the sine law twice to get:
$$\frac{AD}{CD} = \frac{\sin 30^\circ}{\sin 15^\circ} = \frac{\sin(135^\circ -\widehat{BAD})}{\sin \widehat{BAD}} $$