(b) : Number of Y atoms per unit cell in ccp lattice
(N) = 4
number of tetrahedral voids = 2N = 2 × 4 = 8
number of tetrahedral voids occupied by X
= 2/3rd of the tetrahedral void = 2/3 × 8 = 16/3
Hence the formula of the compound will be
X16/3 Y4 = X4 Y3