Atoms ‘X’  are in ccp arrangement. Atoms ‘Y’ occupy one third of the tetrahedral voids. Formula of the compound is

For CCP unit cell; Z = 4(n)number of TV/unit cell = 8(2n)number of OV/unit cell = 4(n)∴number of atoms of ‘X’ = n    number of atoms of ‘Y’ = 2n×⅓Formula of the compound = X3Y2