WAEC Mathematics 2013 Paper

Concept:For any regular polygon, each interior angle and its adjacent exterior angle form a straight line, so their sum is $180^\circ$. The number of sides is found by dividing $360^\circ$ by the exterior angle.Explanation:Let each exterior angle be $x$.Then the interior angle is $5x$.Since interior angle + exterior angle $= 180^\circ$,$5x + x = 180^\circ$$6x = 180^\circ$$x = 30^\circ$So each exterior angle is $30^\circ$.Number of sides $= \frac{360^\circ}{\text{exterior angle}}$Number of sides $= \frac{360^\circ}{30^\circ} = 12$Answer:The polygon has 12 sides.Option B.