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

In an $FCC$ unit cell,what fraction of the edge length is not covered by atoms?

The fraction of total volume occupied by the atoms present in a simple cubic unit cell is

$A$ compound is formed by atoms of $A$,$B$ and $C$. Atoms of $C$ form $hcp$ lattice. Atoms of $A$ occupy $50\%$ of octahedral voids and atoms of $B$ occupy $\frac{2}{3}$ of tetrahedral voids. What is the molecular formula of the solid?

$A$ crystalline solid is made of $X$,$Y$ and $Z$ elements. Atoms of $X$ form $FCC$ packing; atoms of $Y$ occupy octahedral voids while atoms of $Z$ occupy tetrahedral voids. What will be the simplest formula of the solid if atoms along one body diagonal are removed?

$Ag$ crystallises in $fcc$ lattice. What is the total number of tetrahedral voids present in $540 \ g$ of $Ag$ metal (in $N_A$)? $(N_A = \text{Avogadro number}; Ag \text{ atomic weight} = 108 \ g \ mol^{-1})$