JAMB 2001 · UME · Q21

Find the number of sides of a regular polygon whose interior angle is twice the exterior angle.

Worked solution (try it first)

Let the exterior angle be $e$ .
The interior angle is $2e$ , and the two add up to $180^\circ$ : $3e = 180^\circ$ , so $e = 60^\circ$ .
The exterior angles add up to $360^\circ$ , so the number of sides is $360 \div 60 = 6$ , option C.