Co-interior Angles on Parallel Lines

Co-interior angles lie between two lines, on the same side of a transversal. In NSW Year 7 maths (NESA outcome MA4‑ANG‑C‑01) you learn that on parallel lines they are supplementary, adding to \(180^\circ\), rather than equal.

You will recognise the C shape, find unknown angles by subtracting from \(180^\circ\), and fix the common mistake of treating co-interior angles as equal. Some questions use algebra: co-interior angles of \(x^\circ\) and \(2x^\circ\) give \(3x = 180\), so \(x = 60\). Shelves fixed between two parallel walls are a real example. This subtopic is part of Year 7 Angles.