Number of N ion in each fcc unit cell = 4
Number of tetrahedral voids =$4\times 2=8$
Fraction occupied by $(M^+)$ ions =$8\times \large\frac{1}{3}=\frac{8}{3}$
Ratio of N : M = 4 : $\large\frac{8}{3}$
$\Rightarrow 4\times \large\frac{3}{8}$
$N : M = 3 : 2= M_2N_3$
Coordination number of $M^+$ ion = 4