According to the theorem, the corresponding angles, formed by a transversal on a pair of parallel sides, are always equal.
Also, the sum of angles on a straight line is 180 degree.
The angles 5 and 7 constitute a pair of corresponding angles, formed by the transversal 'r' on the pair of parallel sides 'p' and 'q'. So they must be equal,
[tex]\begin{gathered} \angle5=\angle7 \\ \angle5=131^{\circ} \end{gathered}[/tex]
The angles 4 and 5 constitute a straight line, so they must add up to be 180 degrees,
[tex]\begin{gathered} \angle4+\angle5=180^{\circ} \\ \angle4+131^{\circ}=180^{\circ} \\ \angle4=180^{\circ}-131^{\circ} \\ \angle4=49^{\circ} \end{gathered}[/tex]
Thus, the angle 4 measures 49 degrees.
