${{50}^{\circ }}+{{108}^{\circ }}+{{90}^{\circ }}+{{40}^{\circ }}={{288}^{\circ }}$
${{360}^{\circ }}-{{288}^{\circ }}={{72}^{\circ }}$
$\frac{72}{360}=\frac{\bigcirc }{100}\Rightarrow \bigcirc =\frac{100\times 72}{360}=20{\scriptstyle{}^{0}/{}_{0}}$