A binary solid (A+B-) has a zinc blende structure with B- ions constituting the lattice and A+ ions occupying 25% of the tetrahedral holes. The formula of the solid is

ZnS structure is F.C.C structure. The number of lattice points per unit cell = 4B-  occupy lattice points =4 Number  of Tetrahedral voids  = 2×4=8A+ occupy 25% of tetrahedral voids25% of tetrahedral voids are occupied =8×25100=2So the required formula is  AB2