An angle is one-fifth of its supplement. The measure of the angle is

(b) 30°

Let the measure of the required angle be $x^{\circ}$

Then, the measure of its supplement will be $(180-x)^{\circ}$

$\therefore x=\frac{1}{5}\left(180^{\circ}-x\right)$

$\Rightarrow 5 x=180^{\circ}-x$

$\Rightarrow 6 x=180^{\circ}$

$\Rightarrow x=30^{\circ}$