A cuboid of dimension  $2\mathrm{L}\times 2\mathrm{L}\times \mathrm{L},$ a charge q is placed at centre of surface with area $4{\mathrm{L}}^2$. The flux through the opposite surface to ‘S’ is given by.

We can put similar cuboid on above this cuboid to make cube of side 2L and to have symmetry about charge 

Flux through this whole cube $=\frac{q}{{\epsilon }_0}$

As total faces are 6 , so flux through ABCD is $\frac{q}{6{\epsilon }_0}$