Maths questions? help. Cant do it :s?

Ok, since the exterior angle and interior angle form a line, their angles add up to 180 deg. 180-18=162 deg for each angle in the polygon. deg of ang x number of angles=180(# of angles-2) so 162a=180(a-2) .9a=a-2 .1a=2 a=20 so there are 20 angles in the polygon. amount of angles=amount of sides so the polygon has 20 sides! :)