A point charge q is placed at P(0,0,a). THe electric flux through a surface subtended by A(0,a,0), B(a,0,0) and C(a,a,0) due to the electric field of q is??

If we make a cube of side a from the origin, the surface subtended by A,B,C forms a face of the cube and the charge q sits on one of the edges. We assume an arrangements of cube, then, q is shared with 8 cubes which form a closed surface.

so flux through one of the cube

and hence through one of the faces of the cube