In cyclic quadrilateral WXYZ, angle ZYW = 10 degrees and angle YOW = 100 degrees. Find angle YWZ.
Since angle(YOW) = 100o, arc(YZW) = 100o.
Since angle(ZYW) = 10o, arc(ZW) = 20o.
Subtracting, arc(YZ) = 80o.
Angle(YWZ) = ½Arc(YZ) = 40o.