How do you find the size of the interior angles of a 12-gon?

Assuming you mean a regular

12-gon:

sum(exterior_angles) = 360o
→ exterior_angle x number_of_sides = 360o (since for a regular polygon they are all the same size).
→ exterior_angle = 360o ÷ number_of_sides
interior_angle + exterior_angle = 180o
→ interior_angle = 180o - exterior_angle
= 180o - (360o ÷ number_of_sides)
= 180o - (360o ÷ 12)
= 180o - 30o
= 150o