DRDO Multi tasking staff Model paper 7

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°