A compound made up of elements A and B (with a general formula $A_x{B}_y$) where B form a hcp lattice and A occupy $\frac{2}{3}$ of the tetrahedral voids. The formula of the compound is

Number of B atoms in lattice is 6.

$\frac{1}{6}\times 12+\frac{1}{2}\times 2+1\times 3=6$

Number of tetrahedral voids is twice the number of atoms that required to form lattice (or number of B atoms).

$2\times 6=12$

Number of atoms, A is:

$\frac{2}{3}\times 12=8$

Formula becomes: 

${\mathrm{A}}_8{\mathrm{B}}_6={\mathrm{A}}_4{\mathrm{B}}_3$