Question

Find the measure of an angle that is one half the measure of its supplement

Answer 1

Let the angle be x. Then another way of saying this is that it's supplement is twice its measure.
So, its supplement can be expressed as 2x.
Adding the two angles, x and 2x, we have
x + 2x = 180
3x = 180
x = 60
The angle is 60 degrees.

(Its supplement is 120, and 60+120=180.)