Each internal angle measures 108 degrees

A triangle has an internal angle of 180/3 A square has an internal angle of 360/4 A pentagon has an internal angle of 540/5 An 'x'-agon has an internal angle (180(x-2))/x = (180x-360)/x = 180 - 360/x A 20-agon has an internal angle of 180 - 360/20 = 162

yes.

An obtuse angled triangle can have can have one internal angle of 120 degrees.

The smallest internal angle possible for a regular polygon is 60 degrees. So either the angle is not 12 degrees or it is not an internal angle.