Find the measure of an exterior angle of a regular polygon with 20 sides. round to the nearest tenth ?

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

Exterior angles always add up to 360°

Given that there are 20 sides sides
⇒ 1 exterior angle = 360 ÷ 20 = 18°

18°