The number of octahedral and tetrahedral holes respectively present in a hexagonal close packed $(hcp)$ crystal of $X$ atoms are

What is the percentage efficiency of packing in $BCC$ structure (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 $= \dots \dots \%$.

What is the percentage of unoccupied volume in $BCC$ structure (in $\%$)?

Calculate the volume occupied by a particle in $fcc$ unit cell if the volume of the unit cell is $1.6 \times 10^{-23} \ cm^3$.

$A$ solid element (metal) crystallises in $ABABAB$ type packing in three dimensions. If the density of the element and its atomic radius are $5.0 \ g/cm^3$ and $100\sqrt{2} \ pm$ respectively, the atomic mass of the element is $(N_A = 6 \times 10^{23} \ mol^{-1})$