A compound is formed by two elements X and Y . The element Y forms...

Atoms of Element $Y$, are present in cubic close packing. In cubic close packing the number of atoms per unit cell is $4$.

So, the effective number of element $Y$, per unit cell is $4$

Element $X$, are present at one third of the tetrahedral voids.

Number of tetrahedral void =Twice the number of atoms in unit cell

Number of atoms of Element $X$ are present unit cell $= \dfrac{1}{3} \times 8 = \dfrac{8}{3}$

$X: Y\dfrac{8}{3}: 42: 3$

Formula of the compound $= {X}_{2}{Y}_{3}$