In a cubical unit cell:
Contribution for a corner particle is =18
Contribution for a face centre particle is =12
Contribution for a edge centre particle is =14
Contribution for a body centre particle is =1
When a one body diagonal is removed, two atoms at corners (here X) and one atom at body center (here Z) is removed.
When another body diagonal is removed, only two more corner atoms are removed, since body centre particle is already removed.
For X : Since 4 corner particles are removed.
Total X atoms in a unit cell=(4×18)=12
For Y : No particle removed.
Total Y atoms in a unit cell=(6×12)=3
For Z : Body centre removed.
Total Z atoms in a unit cell=(12×14)+0=3
Formula for the compound:
X12Y3Z3Simplest formula : XY6Z6