What are the percentages of empty space in a cubic close-packed $(CCP)$ structure and a body-centered cubic $(BCC)$ structure,respectively?

$A$ solid has an $hcp$ lattice. Atoms of $Z$ (anions) form the $hcp$ lattice. Atoms of $X$ (cations) occupy all the octahedral voids in the lattice. Atoms of $Y$ (cations) occupy half of the tetrahedral voids. What is the molecular formula of the solid?

The number of tetrahedral voids in the unit cell of a face-centered cubic $(fcc)$ lattice of similar atoms is:

What is the packing efficiency of silver metal in its unit cell (in $\%$)?

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 $A_{x} B_{y}$?

What is the percentage of space occupied by spheres of equal size in $hcp$ and $ccp$ three-dimensional arrangements?