The packing efficiency of the face-centered cubic $(fcc)$,body-centered cubic $(bcc)$,and simple/primitive cubic $(pc)$ lattices follows the order:

Consider the $bcc$ unit cells of the solid $1$ and $2$ with the position of atoms as shown below. The radius of atom $B$ is twice that of atom $A$. The unit cell edge length is $50\%$ more in solid $2$ than in $1$. What is the approximate packing efficiency in solid $2$? $........... \%$

In the closest packing of atoms,

Element '$B$' forms $ccp$ structure and '$A$' occupies half of the octahedral voids,while oxygen atoms occupy all the tetrahedral voids. The structure of bimetallic oxide is

The atomic radius of an $fcc$ unit cell is:

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