JAMB 1986 · UME · Q31

A regular polygon of $n$ sides has $160^\circ$ as the size of each interior angle. Find $n$ .

Worked solution (try it first)

An interior angle and its exterior angle add up to $180^\circ$ , so each exterior angle is $180^\circ - 160^\circ = 20^\circ$ .
The exterior angles add up to $360^\circ$ , so $n = 360 \div 20 = 18$ , option A.