What is the measure of an angle whose supplement is three times its measure?

let x = measure of the mystery angle and y = measure of its supplement

By definition of supplement, x+y = 180 since any angle and its supplement sum to a straight angle

The problem gives us that 3x = y

Substituting in 3x for y in the supplement equation gives us

x + 3x = 180

4x = 180

x = 180/4

x = 45

Check our answer: The supplement of 45 degrees is 180 - 45 = 135. 45*3 = 135. Therefore, the answer is 45 degrees.