WAEC Mathematics 2013 Paper

Concept:Use the angle properties of a triangle and relations between angles formed by intersecting and parallel lines.Explanation:In $\triangle ABC$, angle $A$ and the given $153^\circ$ lie on a straight line.So, $\angle A = 180^\circ - 153^\circ = 27^\circ$.The angle at $B$ is marked as $2m$, and it is vertically opposite to another angle, so $\angle B = 2m$.The angle at $C$ is marked as $7m$ because the lines form corresponding angles, so $\angle C = 7m$.Using the angle sum of a triangle:$\angle A + \angle B + \angle C = 180^\circ$$27 + 2m + 7m = 180$$27 + 9m = 180$$9m = 153$$m = \frac{153}{9} = 17$Answer:$m = 17^\circ$, which is option D.