In $\mathrm{C}\mathrm{r}\mathrm{C}{\mathrm{l}}_3$ for every, $3\mathrm{C}{\mathrm{l}}^{-}$ ions, there is one $\mathrm{C}{\mathrm{r}}^{3+}$ ion is present.

Thus, if there are $3\mathrm{C}{\mathrm{l}}^{-}$ ions are present in CCP.

Number of octahedral voids = 3, and tetrahedral voids = 6

Total voids = 3+6=9

As $\mathrm{1 }\mathrm{C}{\mathrm{r}}^{3+}$ ion is present in the octahedral void, therefore, fraction of holes occupied $=\frac{1}{9}$