Let interior and exterior angle of polygon be I and E respectively. Then, I - E = 60° ----(1) ∵ The sum of interior and exterior angle is 180° ∴ I + E = 180° ----(2) Solving (1) and (2) for I and E, ⇒ I = 120° and E = 60° Let n be the number of sides of polygon ∴ 60° × n = 360° (∵ sum of exterior angles of any regular polygon is 360°) ⇒ n = 6 ∴ sum of interior angles = (n - 2) × 180° = (6 - 2) × 180° = 720°