In a crystalline solid, having formula XY_2 O_4 , oxide ions are...

In a cubic close packed lattice of oxide ions, there would be two tetrahedral and one octahedral void per oxide ion. Since, the formula shows the presence of 4 oxide ions, the number of tetrahedral voids is eight and that of octahedral voids is four. Out of the eight tetrahedral voids, one is occupied by X . $\therefore$ Percentage of tetrahedral voids occupied $=\frac{1}{8} \times 100=12.5 \%$