S1.2 In Figure $1, A D$ and $B E$ are straight lines with $A B=A C$ and $A B / / E D$.
If $\angle A B C=R^{\circ}$ and $\angle A D E=S^{\circ}$, find the value of $S$.
$\begin{array}{ll}\angle A B C=30^{\circ}=\angle A C B & (\text { base } \angle \text { isos. } \triangle) \\ \angle B A C=120^{\circ} & (\angle \text { s sum of } \triangle) \\ \angle A D E=120^{\circ} & (\text { alt. } \angle \mathrm{s} A B / / E D) \\ S=120 & \end{array}$