To find the interior angle, you use the formula ((n - 2) x 180) / n (where n is the number of sides).
So, ((20 - 2) x 180) / 20 = 162
To find the exterior angle, you merely take away the interior angle from 180.
180 - 162 = 18, so the exterior angle is 18 degrees
Another, easier, way is to just divide 360 by n. All exterior angles always add to 360. Thus, 360 / 20 = 18 degrees
