The radius ratio of cation to anion for a triangular void 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?

Fill in the blanks:
$(a)$ In $ccp$ structure,$a = \dots \dots r$.
$(b)$ In $hcp$ structure,packing efficiency $= \dots \dots \%$.
$(c)$ In $bcc$ structure,percentage of packing efficiency $= \dots \dots \%$.

Percentages of free space in cubic close packed structure and in body centered packed structure are respectively

$A$ 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 $N_{A} = 6.02 \times 10^{23}$)

In a solid,$B^{-}$ ions occupy corners of a cube forming $ccp$ structure. If $A^{+}$ ions occupy half the tetrahedral voids,the formula of the solid is: