Angles 1, 2, and 3 are supplementary (they add up to equal 180). If angles 1 and 3 are 20 each, then their sum in 40, making angle 2
180 - 40 = 140. If this central angle is 140 degrees, then the measure of the arc that it cuts off also measures 140 degrees. This is not to be confused with the arc length, which is not even remotely the same thing!
