let the exterior angle be x. to find the sum of all interior angles we use the formula: (n-2)x 180. here n=18, so (18-2) x 180 = 2880
The sum of all interior angles is 2880 so the measure of 1 interior angle will be : 2880/18 = 160 degree.
now we know the measure of the interior angle then : 160 +x=180 ( as it will form a linear pair)
therefore x= 180-160= 20 degree.
x= 20 degree.
thus the measure of each outer angle is 20 degree.
