A crystal is made of particles X, Y and Z. X forms FCC packing. Y occupies all the octahedral voids of X and Z occupies all the tetrahedral voids of X. If all the particles along one body diagonal are removed then the formula of the crystal would be:

No. of X particles per unit cel = 4No. of Y particles = 4

No. of Z particles = 8 Along one body diagonal 2X atoms from 2 corners, one Y particle and 2Z particles will be removed.
So, effective no. of X particles in a unit cell $=4-\frac{1}{8}\times 2=\frac{15}{4}$

Effective no. of Y particles in a unit cell $=4-1=3$

Effective no. of Z particles in a unit cell $=8-2=6$

$\begin{array}{rr} & \mathrm{X}:\mathrm{Y}:\mathrm{Z}\\ & \frac{15}{4}:3:6\\ & 5:4:8\end{array}$