$A$ metal $M$ forms a hexagonal close-packed structure. The total number of voids in $0.02 \ mol$ of it is $......... \times 10^{21}$ (Nearest integer) (Given $N_{A} = 6.02 \times 10^{23}$)

Which one of the following statements about packing in solids is incorrect?

In an ionic oxide,the oxide anions form an $hcp$ arrangement,while the metal cations occupy $\frac{2}{3}$ of the octahedral voids. Determine the formula of this oxide.

$Ga$ (atomic mass $70 \, u$) crystallizes in a hexagonal close packed structure. The total number of voids in $0.581 \, g$ of $Ga$ is $\dots \dots \dots \dots \dots \times 10^{21}$. (Round off to the Nearest Integer).

$A$ solid compound is formed by atoms of $A$ (cations),$B$ (cations),and $O$ (anions). Atoms of $O$ form an $hcp$ lattice. Atoms of $A$ occupy $25\%$ of the tetrahedral voids,and atoms of $B$ occupy $50\%$ of the octahedral voids. What is the molecular formula of the solid?

With the help of a labelled diagram,show that there are $4$ octahedral voids per unit cell in a cubic close-packed $(CCP)$ structure.