$20{}^\circ =\frac{\overset\frown{AB}-\overset\frown{CD}}{2}\Rightarrow 40{}^\circ =3\overset\frown{CD}-\overset\frown{CD}\Rightarrow 40{}^\circ =2\overset\frown{CD}\Rightarrow \overset\frown{CD}=20{}^\circ $