Question:

What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 1/3rd of tetrahedral voids?

No. of Y ccp atom = 1
No. of tetrahedral void = 2
No. of X atom = 2 × 1/3 = 2/3
The ratio of X : Y = 2/3 : 1
Formula of compound : X₂Y₃