Question

The pair of angles that adds up to a straight angle are called angles.

Answer 1

The pair of angles that add up to a straight angle are called supplementary angles.

For example, consider the angles ${120}^o$ and ${60}^o.$
${120}^o+{60}^o={180}^o$
${180}^o$ is a straight angle.

$\therefore {120}^o$ and ${60}^o$ are a pair of supplementary angles.

Supplementary angles are the pair of angles that add up to ${180}^o.$