We can start with the statement that the sum of all exterior angles of a polygon will add 360 degrees.
For example, for the quadrilateral (square):
Then, each exterior angle must have a value of 360/n.
n is the number of sides.
In the case of the square, n is 4.
For a pentagon, n=5.
The interior angles are supplementary of the exterior angles, so they have a value of:

For a quadrilateral the measure of the interior angle is 90 degrees:

For a pentagon (n=5), the measure of the interior angle is 108 degrees.

For a dodecagon (n=12), we have a measure of 150 degrees for the interior angle:
