Number of effective atoms in HCP lattice is =6

Number of atoms of $\mathrm{X}=6$

Number of T.V. in h.c.p. =$12 \left(2\times \mathrm{Number} \mathrm{of} \mathrm{atoms}\right)$

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

$\%$ of $\mathrm{X}=\frac{6}{14}\times 100=42.86\%$

Round off to the nearest integer = 43