In a solid 'XY' having the NaCl structure, 'X' atoms occupy the...

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 $= 8 \times \dfrac{1}{8} + 4 \times \dfrac{1}{2} = 3$
Number of Y atoms per unit cell $= 12 \times \dfrac{1}{4} + 1 = 4$
Thus, formula is ${X}_{3}{Y}_{4}$ .
Hence, m = 3, n = 4
sum of m + n = 3 + 4 = 7.