In a closed packed array of $N$ spheres,the number of tetrahedral holes is

In a face-centred cubic arrangement of $A$ and $B$ atoms,$A$ atoms are at the corners of the unit cell and $B$ atoms are at the face centres. If three of the $B$ atoms are missing from the face centres in the unit cell,what is the simplest formula of the compound?

What is the void volume in the crystal lattice formed by a $BCC$ unit cell (in $\%$)?

In a close-packed lattice,how does the size of an octahedral void compare to that of a tetrahedral void?

If $Z$ is the number of atoms in the unit cell that represents the closest packing sequence $ABCABC .....,$ the number of tetrahedral voids in the unit cell is

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 $= \dots \dots \%$.