What is the formula of compound in which the element Y forms ccp lattice and atoms of X occupy 2/3rd of tetrahedral voids?

Effective no of atoms forming ccp is X = 4.
Effective number of tetrahedral holes in ccp is Y = 8.
So, 8 × 2/3 = 16/3
X : Y = 4 : 16/3 = 12 : 16 = 3 : 4
So, the formula is X₃Y₄.