A binary solid $\left({\mathrm{A}}^{+}{\mathrm{B}}^{-}\right)$ 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 = 4

B^-  occupy lattice points =4

 Number  of Tetrahedral voids  = $2\times 4=8$

A⁺ occupy 25% of tetrahedral voids

25% of tetrahedral voids are occupied =8$\times \frac{25}{100}=2$

So the required formula is  AB₂