A compound of formula ${\mathrm{A}}_2{\mathrm{B}}_3$ has the $\text{hcp}$ lattice. Which atom forms the $\mathrm{hcp}$ lattice and what fraction of tetrahedral voids are occupied by the other atoms

A solid having density of $9\times {10}^3\mathrm{kg} {\mathrm{m}}^{-3}$ forms face centred cubic crystals of edge length $200\sqrt{2}\mathrm{pm}$. What is the molar mass of the solid? Avogadro's constant ${\text{≅6×10}}^{23}{\mathrm{mol}}^{-1},\mathrm{\pi }\cong 3.14$

The radius of the largest sphere which fits properly at the centre of the edge of a body-centred cubic unit cell is:(Edge length is represented by $\text{'}\mathrm{a}\text{'}$)