$\mathrm{KCl}$ crystallises in the same type of lattice as does $\mathrm{NaCl}$. Given that ${\mathrm{r}}_{{\mathrm{Na}}^{+}}/{\mathrm{r}}_{{\mathrm{Cl}}^{-}}=0.55$ and ${\mathrm{r}}_{{\mathrm{K}}^{+}}/{\mathrm{r}}_{{\mathrm{Cl}}^{-}}=0.74$. Calculate the ratio of the side of the unit cell of $\mathrm{KCl}$ to that of $\mathrm{NaCl}$.

A metal has cubic close-packed ($\mathrm{CCP}$) arrangement, the layer sequence of which is shown below:

A face diagonal passes through the centre of $4$ atoms and the centre (s) of which other atoms?