How many total voids are present in $1 \ mol$ of a compound that forms an $hcp$ structure?

$A$ metal oxide crystallises in a hexagonal close-packed $(HCP)$ array of oxide ions with two out of every three octahedral holes occupied by metal ions. The formula of the metal oxide is:

What are $ABAB...$ packing and $ABCABC...$ packing respectively called?

$A$ compound formed by $Mg$,$Al$,and $O$ is found to have a cubic close-packed $(CCP)$ array of oxide ions,in which $Mg^{2+}$ ions occupy $\frac{1}{8}^{th}$ of the tetrahedral voids and $Al^{3+}$ ions occupy $\frac{1}{2}$ of the octahedral voids. The formula for the compound is:

$A$ compound is formed by elements $A$ and $B$. Element $B$ forms an $HCP$ lattice,and element $A$ occupies $2/3$ of the tetrahedral voids. What is the formula of the compound formed by $A$ and $B$?

Atoms of element $A$ occupy $\frac{1}{3}$ of the octahedral voids,while atoms of element $B$ form an $hcp$ structure. Determine the formula of the compound formed by $A$ and $B$.