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

$A$ compound formed by $Mg$,$Al$,and $O$ is found to have a cubic close-packed $(CCP)$ array of oxide ions,in which $Mg^{2+}$ ions occupy $\frac{1}{8}^{th}$ of the tetrahedral voids and $Al^{3+}$ ions occupy $\frac{1}{2}$ of the octahedral voids. The formula for the compound is:

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

In a close-packed lattice,the number of tetrahedral voids compared to the number of octahedral voids is:

Calculate the packing efficiency in a simple cubic unit cell.

Atoms of element $X$ form $hcp$ lattice and those of element $Y$ occupy $\frac{2}{3}$ of its tetrahedral voids. The percentage of element $X$ in the lattice is ..... . (Nearest integer)