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

Suppose the number of B- ions that constitute the lattice is 100.The number of tetrahedral sites = 200.Of these, 25% are occupied by A+. Therefore, the number of A+ = 50.So, the ratio of A+ : B-  = 50 : 100 = 1 : 2.The formula is AB2.