WAEC 2023 · Paper 1 · Q21

Find the value of $m$ in the diagram, where the two horizontal lines are parallel.

Worked solution (try it first)

Draw a line through the right-angled corner parallel to the other two.
By alternate angles, the lower arm makes $40^\circ$ with it.
The corner is $90^\circ$ , so the upper arm makes $90^\circ - 40^\circ = 50^\circ$ with the new line.
$m$ and that $50^\circ$ are co-interior between the new line and the upper parallel: $m = 180^\circ - 50^\circ = 130^\circ$ , option B.