JAMB 1990 · UME · Q36

Each interior angle of a regular polygon is $140^\circ$ . How many sides has the polygon?

Worked solution (try it first)

An interior angle and its exterior angle add up to $180^\circ$ , so each exterior angle is $180^\circ - 140^\circ = 40^\circ$ .
The exterior angles add up to $360^\circ$ , so the number of sides is $360 \div 40 = 9$ , option A.