What do triangles and angles on a straight line have in common?
Angles in a triangle and angles on a straight line add up to 180°
Here is a further fact about 180°
The interior angles are within parallel lines and on the same side of the transversal (that line drawn through them)
These angles are sometimes called co-interior or supplementary angles.
There are very few numbers we have to remember in maths 180 is one of them.
Great stuff three facts for the price of one.
To calculate a missing interior angle just subtract the angle given from 180°