Solution :

Suppose atoms of element N present in $CCP= x$

Number of tetrahedral voids $=2x$

Since $\large\frac{1}{3} $ rd of the tetrahedral voids are occupied by atoms of element M.

Number of atoms of elements $M= \large\frac{1}{3}$$\times 2x$

$\qquad= \large\frac{2x}{3}$

Ratio of $M:N=\large\frac{2x}{3}$$:x$

$\qquad=2:3$

Hence the formula of the compound $= M_2N_3$