Na in NaCl has eight corner and six face atoms and thus, if we remove face centred atom of one axis, then two faces atoms are removed. Thus, X is at 8 corners and Y is at 4 faces.
Number of X atoms per unit cell $=8\times \frac{1}{8}+4\times \frac{1}{2}=3$
Number of Y atoms per unit cell $=12\times \frac{1}{4}+1=4$
Thus, formula is ${\mathrm{X}}_3{\mathrm{Y}}_4$ .
Hence, m = 3, n = 4
sum of m + n = 3 + 4 = 7.