91Ó°ÊÓ

The sum of measurement of angles that are positioned on interior side is equal to $180°$when the lines related to the angles are having a parallel relation.

Draw a line S that is having a parallel relation with existing parallel lines.

According to the obtained figure the lines XR and US are having a parallel relation and a separate line RS is travelling through them so according to the method write the measurement of $∠1$.

$∠1+110°=180°$

$∠1=180°−110°$

$=70°$

According to the obtained figure the lines YT and US are having a parallel relation and a separate line ST is travelling through them so according to the method write the measurement of $∠2$.

$∠2+120°=180°$

$∠2=180°−120°$

$=60°$

According to the obtained figure $∠RST$is the addition of $∠1$and$∠2$ so determine the measurement of $∠RST$.

$∠RST=∠1+∠2$

$=70°+60°$

$=130°$