What is the coordination number of a particle in $hcp$ structure?

The crystal structure of the solid compound $XY$ is similar to $NaCl$. If the radius of the cation $X^+$ is $100 \, pm$, then the radius of the anion $Y^-$ will be ......... $pm$.

Consider an ionic solid $MX$ with $NaCl$ structure. Create a new unit cell $(Z)$ from the unit cell of $MX$ by following the sequential instructions given below. Ignore charge balance.
$(i)$ Remove all anions $(X)$ except the central one.
$(ii)$ Replace all face-centered cations $(M)$ with anions $(X)$.
$(iii)$ Remove all cations $(M)$ at the corners.
$(iv)$ Replace the central anion $(X)$ with a cation $(M)$.
The value of $\left(\frac{\text{number of anions}}{\text{number of cations}}\right)$ in $Z$ is . . . . .

$A$ compound formed by elements $X$ and $Y$ crystallises in a cubic structure, where $X$ atoms are at the corners of a cube and $Y$ atoms are at the body centre. The formula of the compound is:

What is the total number of atoms in a $BCC$ crystal lattice having $1.8 \times 10^{20}$ unit cells?

In an ionic crystal,$\frac{r^{+}}{r^{-}} = 0.524$,the coordination number of the cation is: