Element 'B' forms ccp structure and 'A' 'occupies half of the...

No. of lattice points = No. of octahedral voids $=\frac{1}{2} \times$ No. of tetrahedral voids in $c c p$ structure $\therefore$ No. of atoms of $B=4$ No. of atoms of $A=\frac{1}{2} \times$ No. of octahedral voids $=\frac{1}{2} \times 4=2$ No. of atoms of $O=$ All tetrahedral voids $=2 \times$ No. of lattice points $=2 \times 4=8$ Hence, $A: B: O=1: 2: 4$ Therefore, the formula of the compound is $A B_{2} O_{4}$