If the given angle is 20 degrees then the other unknown angle would also be 20 degrees because this triangle is an isosceles triangle. 20+20+20=180 proving the triangle sum theorem.
The only given number here is the 20 degree angle. So we'll start with that. This is an inscribed angle, which doubles to 2*20 = 40, and this is the measure of the arc the inscribed angle cuts off (inscribed angle theorem). Consequently, it means that central angle b is also 40 degrees.
With b = 40, we can see that c = 180-b = 180-40 = 140. This is because b+c = 180 as the two angles are supplementary.
