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

Calculate the void volume of a simple cubic unit cell if the volume of the unit cell is $5.5 \times 10^{-22} \ cm^3$.

If the anion $(A)$ forms a hexagonal closest packing and the cation $(C)$ occupies only $2/3$ of the octahedral voids in it,then the general formula of the compound is :-

The percentage of vacant space for $bcc$ unit cell is ............. $\%$

$A$ compound is formed by two elements $A$ and $B$. Atoms of the element $B$ (as anion) make $ccp$ lattice and those of element $A$ (as cation) occupy all tetrahedral voids. The formula of the compound is

$A$ compound forms a hexagonal close-packed $(HCP)$ structure. How many octahedral voids are present in $0.3 \, mol$ of it?