The total flux through the faces of the cube with side of length $a$, if a charge $q$ is placed at corner $A$ of the cube is

In the figure, when a charge $q$ is placed at corner $A$ of the cube, so 8 such cubes will completely enclose this charge.

$\therefore$ The total flux through the faces of one such cube $=\frac{q}{8{\epsilon }_0}$