Ga (atomic mass) crystallizes in a hexagonal close packed structure. The total number of voids in 0.581 g of Ga is ______ × 10²¹.
(Round off to the Nearest Integer)

HCP crystallises from Ga.
If the amount of particles per cell is n,
Tetrahedron void count = 2n
Octahedron void count  = n
                                Total  = 3n
The number of provided gallium atoms or particles is = $\frac{\mathrm{W}}{\mathrm{GAW}}\times {\mathrm{N}}_{\mathrm{A}}=\frac{0.581}{70}\times 6.023\times {10}^{23}=0.0499\times {10}^{23}\text{ atom }$
-3 voids exist for each particle.
$$\begin{gathered} 0.0499\times {10}^{23}= ? \\ \frac{0.0499\times {10}^{23}}{1}\times 3=0.1499\times {10}^{23}\text{ voids } \\ 14.99\times {10}^{21}\cong 15\times {10}^{21}\text{ voids } \\ a = 15 \end{gathered}$$

Therefore, the correct answer is 15