A charge Q is placed at a distance a/2 above the centre of the square surface of edge a as shown in the figure. The electric flux through the square surface is:

Draw an imaginary cube enclosing that point charge at the centre. Then from Gauss's law, Φ = Q/ε₀. This is the flux passing through all six surfaces. Therefore, through one surface, 1/6th of this flux will pass.