A metal M forms hexagonal close-packed structure. The total number...

From the diagram of voids in HCP:

There are 6 octahedral voids and 12 tetrahedral voids in a HCP unit cell.
One unit cell of HCP contains $= 18$ voids

Thus, No. of voids in $0 . 02mol$ of hcp

$= \dfrac{18}{6} \times 6 . 02 \times {10}^{23} \times 0 . 02$

$\approx 3 . 6 \times {10}^{22}$

$\approx 36 \times {10}^{21}$