Angles: parallel lines, polygons, interior/exterior

Students confuse the two and divide $360$ when they meant the interior, or vice versa.

Assigning a value to the wrong angle (e.g. deciding an interior angle is the exterior one) loses every mark even when the method is right.

After finding one angle, students fail to notice two equal sides/angles (e.g. radii or a symmetric shape) that unlock the next step.

Using $(n-2)\times 180$ as a single interior angle — that is the TOTAL of every interior angle. For a regular polygon divide by $n$: a 12-gon gives $\dfrac{1800}{12}=150°$, not $1800°$.

Forgetting that angles on a straight line sum to $180°$ and angles at a point sum to $360°$, so a needed angle in the diagram is never found and the reasoning chain stalls.

Treating a Z-pair (alternate) as adding to $180°$, or a C-pair (co-interior) as equal. Alternate angles are EQUAL; co-interior (allied) angles ADD to $180°$.