So when you wish to find the sum of any polygons interior angles you take the number of sides and minus it by 2. You then times that number by 180 and you get the sum of the interior angles, if you need what all of the angles are individually you divide the sum by how many sides you have. Exterior angles Always equal 360. So your problem is this:
20 - 2 = 18 , 18 × 180 = 3240°
(Divide that number by 20 if you need what each individual angle is)