The percentage of vacant space for $bcc$ unit cell is ............. $\%$

In a compound $XY$,the atoms of element $Y$ form an $fcc$ structure,while the atoms of element $X$ occupy $25\%$ of the tetrahedral voids. The molecular formula of the compound will be .......

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 an ionic crystalline solid,atoms of element $Y$ form an $hcp$ structure. The atoms of element $X$ occupy one-third of the tetrahedral voids. What is the formula of the compound?

The number of tetrahedral voids per unit $HCP$ cell is

Which of the following is the correct order of packing efficiency?