Quadrilateral Inscribed angles calculation with one arc angle I am trying desperately to solve following problem. How can I solve it, the image and question is included in image !enter image description here

Since $\square ABCD$ is a cyclic quadrilateral, $m\angle ADC + m\angle ABC = 180^\circ \implies m\angle ADC = 180^\circ - 100^\circ = 80^\circ$, using this.