Ga (atomic mass 70u ) crystallizes in a hexagonal close packed...

$HCP$ structure : Per atom, there will be one octahedral void $( OV )$ and two tetrahedral voids $( TV )$.

Therefore, total three voids per atom are present in $HCP$ structure.

$\to$ Therefore total no. of atoms of $Ga$ will be-

$= \dfrac{Mass}{MolarMass} \times {N}_{A} = \dfrac{0 . 581g}{70g / mol} \times 6 . 023 \times {10}^{23}$

$\to$ Now, total Number of voids $= 3$ $\times$ total no. of atoms

$= 3 \times \dfrac{0 . 581}{70} \times 6 . 023 \times {10}^{23} = 14 . 99 \times {10}^{21}$

$≃ 15 \times {10}^{21}$