Step-by-step explanation: ok, first off the sides of the polygon THE OUTER sides anyways, measure out to be 360 degrees. You want to divide 360 by 20. So overall for the outer sides you should get 18 sides each being 20 degrees so that means that the interior angle which is half of 360 is 180 divided by half of 20, should be anyways... so the sum of the interior angles is 20-9
