A metal of density $7.5\times 10^3kgm^{-3}$ has an fcc crystal structure with lattice parameter $a=400$pm.Calculate the number of unit cells present in 0.015kg of the metal

Question

$\begin{array}{1 1}2.125\times 10^{22}\\3.125\times 10^{22}\\1.125\times 10^{22}\\4.125\times 10^{22}\end{array} $