What is the two-dimensional coordination number of a molecule in a square close-packed layer?

The number of unit cells present in $39 \ g$ of potassium if it crystallizes as a body-centered cubic $(BCC)$ structure is ($N = \text{Avogadro number}$,$\text{At. wt. of potassium} = 39$).

$NaCl$ is a $FCC$ lattice where $Na^{+}$ ions are at corner and face centre positions. Chloride ions are at edge centres and body centre positions. How many $NaCl$ formula units will be in a unit cell?

The coordination number of an atom in a body-centered cubic $(BCC)$ structure is:
[Assume that the lattice is made up of atoms.]

The number of atoms contained in one face-centred cubic $(FCC)$ unit cell of a monoatomic substance is:

The ratio of anion radius to cation radius of a crystal is $10 : 9.3$. Then,the coordination number of the cation in the crystal is