If an angle is $\frac{3}{5}$ of its supplement. Find the angle

Let the angle be x

so its supplement is $180-x$

according to question

$x=\frac{3}{5}\left(180-x\right)$

$5x=3\left(180-x\right)$

$5x=540-3x$

$$\begin{gathered} 5x+3x=540 \\ 8x=540 \\ x=67.5 \end{gathered}$$

hence, the final answer is $67.5$$°$