Answer:
The sum of the exterior angles must always be 360 deg.
Consider an equilateral triangle 3 * 120 = 360
For a square 4 * 90 = 360
For a pentagon 5 * 72 = 360
So for a 20-gon each interior angle = 180 - 18 = 162 with an exterior angle of 18
One must rotate 18 * 20 to get the full 360 deg so the side is back where it started
Since each interior angle = 162 deg one has
20 * 162 = 3240 deg for the sum of the interior angles