O.N. of P in PO43-, S in SO42- and that of Cr in Cr2O72- are respectively

oxidation number of P in PO4 3- is +5,S in SO4-2   is +6 Cr  in Cr2O7 -2   is +6PO4-3  X=x+4(-2) =-3              =x-8=-3             X  =+5       SO4-2  X+4(-2)=0          X=+8                Cr2O7-2  2X+7(-2)=-2               2X-14=-2                 X=+6