Edge length of the unit cell $=2\stackrel{.}{\mathrm{A}}=2\times {10}^{-8} \mathrm{cm}$

Volume of unit cell $={\left(2\times {10}^{-8}\right)}^3=8\times {10}^{-24} {\mathrm{cm}}^3$

Mass of one unit cell $=$ volume $\times$ density

$=8\times {10}^{-24}\times 25$

Number of unit cell in $200 \mathrm{g}$ of metal

$=\frac{\text{ mass of the metal }}{\text{ mass of one unit cell }}$

$=\frac{200}{8\times {10}^{-24}\times 25}=1\times {10}^{24}$