In an FCC arrangement of A and B atoms, where A atoms are at the corners of the unit cell, B atoms at the face centers, two atoms are missing from two corners in each unit cell, then the simplest formula of a compound is
AB4
Since there are six atoms (A) in the corner of the unit cell, the contribution of atoms in 1 unit cell is 6/8 = 3/4. Since 3 face-centered atoms (B) contribute to one unit cell, the formula is A0.75B3
= A3B12
= AB4.