What is the total volume occupied by atoms in a $bcc$ unit cell (in $\%$)?

$A$ body-centred cubic $(BCC)$ lattice is made up of two different types of atoms $X$ and $Y$. Atom $X$ occupies the body centre and atoms $Y$ occupy the corner positions. One of the corners is left unoccupied per unit cell. The empirical formula of it is

$A$ compound is formed by two elements $X$ and $Y$. Atoms of the element $Y$ (anions) form an $hcp$ structure and atoms of the element $X$ (cations) occupy all the tetrahedral voids. Determine the formula of the compound.

In a compound $XY$,the atoms of element $Y$ form a $ccp$ structure,while the atoms of element $X$ occupy $2/3$ of the tetrahedral voids. The molecular formula of the compound will be ..........

In a $ccp$ arrangement,the percentage of space occupied by spheres is ........ (in $\%$).

Fill in the blanks:
$(a)$ In $ccp$ structure,$a = \dots \dots r$.
$(b)$ In $hcp$ structure,packing efficiency $= \dots \dots \%$.
$(c)$ In $bcc$ structure,percentage of packing efficiency $= \dots \dots \%$.