If the difference between interior and exterior angle of a regular polygon is 60° then what is the sum of interior angles of the polygon?

If the difference between interior and exterior angle of a regular polygon is 60° then what is the sum of interior angles of the polygon? Correct Answer 720°

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° 

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