In a certain regular polygon, the measure of each interior angle is nine times the measure of each exterior angle. Find the number of sides in this regular polygon.
The interior angle and its corresponding exterior angle are supplementary ( sum to 180°)
So.....
9n + n = 180
10n = 180
n = 180 /10 = 18°
And the sum of the measures of the exterior angles = 360°
So....the number of sides = 360 / 18 = 20