#### Answer

The supplement is $75^{\circ}$

#### Work Step by Step

Two angles are supplementary if they add up to $180^{\circ}$.
This means we have to find $\angle A$, where:
$105^{\circ}+\angle A=180^{\circ}$
$\angle A=180^{\circ}-105^{\circ}$
$\angle A=75^{\circ}$