An octahedral void is surrounded by how many spheres?

The packing efficiency of simple cubic $(sc)$, body-centred cubic $(bcc)$ and cubic close packing $(ccp)$ lattices follow the order:

What is the total number of tetrahedral voids in $0.6 \ mole$ of a compound that forms a $hcp$ structure?

For the given close-packed structure of a salt made of cation $X$ and anion $Y$ shown below (ions of only one face are shown for clarity),the packing fraction is approximately
$\text{(packing fraction} = \frac{\text{Packing efficiency}}{100}\text{)}$

Packing fraction of a layer in $2D$ hexagonal close packing arrangement is:

Define: Packing Efficiency.