In a close-packed lattice,how does the size of an octahedral void compare to that of a tetrahedral void?

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.

$A$ compound is formed by elements $X$, $Y$, and $Z$. Atoms of $Z$ (anions) make $FCC$ lattice. Atoms of $X$ (cations) occupy all the octahedral voids. Atoms of $Y$ (cations) occupy $\frac{1}{3}$ rd of the tetrahedral voids. The formula of the compound is:

Explain the formation of voids during the transition from a two-dimensional hexagonal close-packed layer to a three-dimensional close-packed structure.

How many tetrahedral voids are present in $0.4 \ mol$ of a compound that forms an $hcp$ structure?

What is the packing efficiency of $fcc$ crystal structure (in $\%$)?