As at a corner, 8 cubes can be placed symmetrically, flux linked with each cube (due to charge Q at the corner) will be
Q
8ε0
Now for the faces passing through the edge A, electric field E at a face will be parallel to area of face and so flux for these faces will be zero. Now as the cube has six faces and flux linked with three faces (through A) is zero, so flux linked with remaining three faces will be
Q
8ε0
Hence, electric flux passed through all the six faces of the cube is