The sum of all of the interior angles is 3240. This is because a regular 20 sided polygon, which I hope is what a 20-gon is, can be broken into 18 triangles. Because triangles have angle measuring 180 degrees, and because in a regular ploygon all the sides are the same, then the total is 3240. If you have any more questions, don't be afraid to ask them directly, I'll help you if I can. Take care.