The packing efficiency of a simple cubic crystal with an interstitial atom exactly fitting at the body centre is ____

Here the interstitial atom is exactly fitting at the body center
$\begin{array}{l}\therefore {\mathrm{r}}_{+}/{\mathrm{r}}^{-}=0.732\\ \therefore \mathrm{P}.\mathrm{F}=\frac{4}{3}\mathrm{\pi }\frac{{\left({\mathrm{r}}^{+}\right)}^3+4/3\mathrm{\pi }{\left({\mathrm{r}}^{-}\right)}^3}{{\mathrm{a}}^3}\end{array}$
$2{\mathrm{r}}^{+}+2{\mathrm{r}}^{-}=\sqrt{3}\mathrm{a}$ By solving we will get 0.73

The packing efficiency of a simple cubic crystal =73%