So the broken piece $CD = \dfrac{BD}{\sin 30^\circ} = \dfrac{10}{1/2}$ equals—

From $\sin 30^\circ = \frac{BD}{CD}$ with $\sin 30^\circ = \frac{1}{2}$, the broken slanted piece is $CD = \frac{10}{1/2} = 20$ m.