If a solid $A^{+}{B}^{-}$ having ZnS (zinc blends) structure is heated so that the ions along two of the axis passing through the face centre particles are lost and bivalent ion (Z) enters here to maintain the electrical neutrality, so that the new formula unit becomes $A_x{B}_y{Z}_c$ report the value of $\left(x+y+c\right)$.