In an ionic compound the anion $(N^-)$ from cubic close type of packing,while the cation $(M^+)$ ions occupy one third of the tetrahedral voids.Deduce the empirical formula of the compound and the coordination number of $(M^+)$ ions.

Question

$\begin{array}{1 1}M_2N_3,4\\M_3N_3,3\\MN_3,2\\M_2N_2,5\end{array}$