The difference between an interior angle and an exterior angle of a regular polygon is 120°. What is the number of sides of the polygon?

Explanation

Let the interior angle be I and the exterior angle be E. Given I - E = 120° and knowing I + E = 180°, solving these yields 2E = 60°, so E = 30°. The number of sides = 360° / 30° = 12.

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