In a compound oxide ion have ccp arrangement, cations A are present...

Oxide ions are forming the lattice
Effective number of oxide ions $= 4 \left(ccp\right)$
$∴$ Tetrahedral voids $= 2 \times4 = 8$
And effective number of A atom $= \dfrac{1}{8} \times8 = 1$
Number of octahedral voids $= 4$
Effective number of B atom $= \dfrac{1}{2} \times4 = 2$
$∴$ Molecular formulae $= A{B}_{2}{O}_{4}$