${\mathrm{NH}}_4\mathrm{Cl}$ crystallizes in a body-centered cubic type lattice with a unit cell edge length of $387\mathrm{pm}$. The distance between the oppositively charged ions in the lattice is

The density of a pure substance '$\mathrm{A}$' whose atoms are packed in a cubic-close packed arrangement is $1 \mathrm{g} {\mathrm{cc}}^{-1}$. If $\mathrm{B}$ atoms can occupy tetrahedral void and all the tetrahedral voids are occupied by '$\mathrm{B}$' atoms, what is the density of resulting solid in $\mathrm{g} {\mathrm{cc}}^{-1}?$ $[$Atomic mass $\left(\mathrm{A}\right)=30 \mathrm{g} {\mathrm{mol}}^{-1}$ and atomic mass $\left(\mathrm{B}\right)=50 \mathrm{g} {\mathrm{mol}}^{-1}]$.