#### Answer

45

#### Work Step by Step

Let a be the measure of the angle.
By the definition of supplement, the measure of the supplement is $180-a$.
Convert the sentence into an expression.
$a=\frac{1}{3}(180-a)$
multiply each side by 3
$a\times3=\frac{1}{3}(180-a)\times3$
$3a=180-a$
add a to each side
$3a+a=180-a+a$
$4a=180$
divide each side by 4
$4a\div4=180\div4$
$a=45$