According to the condition, the interior angle is 5x, while the exterior angle is 2x, where x is (an unknown) common measure of the two angles. Since interior and exterior angles are supplementary, you have 2x + 5x = 180 degrees, or 7x = 180 degrees, i.e. x =. (1) Next, the sum of exterior angles is 360 degrees: n*(2x) = 360 degrees, or n = . (2) Substitute here x = from (2). You will get n = = = 7. Answer. The number of sides of the polygon is 7.