In a face centred cubic lattice, atom A occupies the corner positions and atom B occupies the face centre positions. If one atom of B is missing from one of the face centred points, the formula of the compound is

Number of effective A atoms
= 8 corners x 1/ 8 per corner atom share
= 1 atoms/unit cell
Number of atoms on faces of a cube = 6 atoms
If one B atom is missing from one face, number of B atoms left = 5
$\therefore$Number of effective ,B atoms = 5 faces x 1/2per face atom
share $=\frac{5}{2}$per unit cell
The formula of the compound is A₂B₅.