A crystal is made of particles X and Y. X forms FCC packing and Y occupies all the octahedral voids. If all the particles along one body diagonal are removed. Then the formula of the crystal would be:
Along one body diagonal 2x atoms from 2 corners, one y particle (ar the centre of cube)
Will be removed.
So effective no of x particles in a unit cell
And effective no of y particles in a unit cell = 4 - 1 = 3
(or) 5 : 4