When two angles form a linear pair, they are adjacent and their non-common sides form a straight line. Therefore, the sum of their measures is always 180°.

Thus: $\text{The sum of } \angle PQR \text{ and } \angle SQR \text{ is } 180^\circ.$

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Related Questions:

1. What is the definition of a linear pair of angles?
2. How do you prove that two angles form a linear pair?
3. If one angle of a linear pair measures $75^\circ$, what is the measure of the other angle?
4. Can two right angles form a linear pair? Why or why not?
5. How do linear pairs relate to supplementary angles?

Tip: Remember, angles that form a linear pair are always supplementary!