Sum of interior anges = ( n-2) * 180
now divide by n to find the measure of each interior angle (n-2)* 180/n
the exterior angles are supplementary ( and exterior is 10 times bigger)
10* ( 180 - (n-2)*180/n) = (n-2)*180/n
1800 - 10(n-2)*180/n = (n-2)*180/n
1800 = 11 (n-2)*180/n
1800n = (11n-22) * 180
-180n = -3960
n = 22 sides