Let the number of sides be " n " (same as the number of vertices). Then we have for the sum of the interior angles 2*120° + (n-2)*150°. From the other side, for each n-gon, the sum of its interior angles is (n-2)*180°. Thus we have this equation 2*120° + (n-2)*150° = (n-2)*180°. Simplify it step by step and find n 2*120 = (n-2)*180 - (n-2)*150 240 = (n-2)*(180-150) 240 = (n-2)*30 n-2 == 8 n = 8 + 2 = 10. ANSWER. n = 10. The polygon is a decagon (10 sides and 10 vertices).