use the formula ext angle = 360/n
Derivation
In any polygon sum of all the interior angles is given by 180(n−2) where n is the number of sides of the polygon and each angle of a regular polygon is given by 180(n−2)/n
measure of ext angle = 180 - int angle
interior angle of a regular polygon is given by 180(n−2)/n
=>=> measure of ext angle =180−(180(n−2))/n
=(180n−180n+360)/n
=360/n
When n=12
Exterior angle =360/12=30
n=15
Exterior angle=360/15=24