G6.3 Each interior angle of an $N$-sided regular polygon is $140^{\circ}$. Find $N$.
Each exterior angle $=40^{\circ}$ (adj. $\angle$ s on st. line $)$
$\frac{360^{\circ}}{N}=40^{\circ} \quad$ (sum of ext. $\angle$ s of polygon)
$$
\Rightarrow N=9
$$