What is the number of octahedral voids present in 0.2 mol of a compound forming hcp structure?

Calculate the volume of simple cubic unit cell if the radius of particle in it is 400 pm.

Calculate the density of a metal molar mass $197 \mathrm{~g} \mathrm{~mol}^{-1}$ if it forms fcc structure. $\left[\mathrm{a}^3 \times \mathrm{N}_{\mathrm{A}}=40 \mathrm{~cm}^3 \mathrm{~mol}^{-1}\right]$

What is minimum number of spheres required of develop a tetrahedral void?