Which arrangement is most suitable for the uniform packing of atoms in a layer?

In a face-centred cubic $(FCC)$ unit cell,what is the total volume occupied by the atoms?

The structure of a mixed oxide is cubic close packed $(ccp)$. The cubic unit cell of the mixed oxide is composed of oxide ions. One-fourth of the tetrahedral voids are occupied by divalent metal $A$ and the octahedral voids are occupied by a monovalent metal $B$. The formula of the oxide is:

The molecular formula of a crystalline solid is $X_3Y_2$. Atoms of $Y$ form $ccp$ lattice,and atoms of $X$ occupy $50\%$ octahedral voids and $x\%$ of tetrahedral voids. What is the percentage of unoccupied tetrahedral voids?

In an ionic solid,anions $(B)$ are arranged in a $ccp$ array and cations $(A)$ occupy $1/3$ of the tetrahedral voids. What is the formula of the ionic compound?

In an $A_{x} B_{y}$ crystal structure,$A$ atoms occupy all octahedral as well as all tetrahedral voids,and $B$ atoms are at the $FCC$ lattice points. What is the formula of the compound $A_{x} B_{y}$?