What is the percentage of unoccupied volume in $BCC$ structure (in $\%$)?

$A$ metal having atomic mass $60.23 \ g/mol$ crystallises in $ABCABC$ close packing. Calculate the density of a single metal atom if the edge length of the unit cell is $10 \ \mathring{A}$. [Given: $N_A = 6.023 \times 10^{23}$]

What is the type of hole occupied if the limiting value of $\frac{r+}{r-}$ is in the range of $0.225$ to $0.414$?

The packing efficiency of simple cubic $(sc)$, body-centred cubic $(bcc)$ and cubic close packing $(ccp)$ lattices follow the order:

$A$ compound is formed by elements $A$, $B$ and $O$. Atoms of oxygen form $ccp$ lattice. Atoms of $A$ (cation) occupy $\frac{1}{8}^{\text{th}}$ of tetrahedral voids and atoms of $B$ (cation) occupy half of octahedral voids. What is the molecular formula of the compound?

Ferric oxide crystallises in a hexagonal close-packed array of oxide ions with two out of every three octahedral holes occupied by ferric ions. Derive the formula of the ferric oxide.