Atoms of element X form hcp lattice and those of element Y occupy...

Number of effective atoms in HCP lattice is =6

Number of atoms of $X = 6$

Number of T.V. in h.c.p. =$12\left(2 \times Numberofatoms\right)$

Number of $Y$ atoms $= \dfrac{2}{3} \times 12 = 8$ atoms.

$$ of $X = \dfrac{6}{14} \times 100 = 42 . 86$

Round off to the nearest integer = 43