In a face-centered cubic lattice, a unit cell is shared equally by how many unit cells?

A compound formed by elements X and Y crystallizes in a cubic structure in which the X atoms are at the corners of a cube and the Y atoms are at the face-centers. The formula of the compound is :-

The pyknometric density of sodium chloride crystal is 2.165 $$ \times $$ 10³ kg m^$$-$$3 while its X-ray density is 2.178 $$ \times $$ 10³ kg m^$$-$$3. The fraction of unoccupied sites in sodium chloride crystal is

When Zn converts from melted state to its solid state, it has hcp structure, then find the number of nearest atoms.