How many spheres form an octahedral void in a close-packed structure?

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?

$A$ 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 packing efficiency of simple cubic $(sc)$,body-centred cubic $(bcc)$ and cubic close packing $(ccp)$ lattices follow the order:

Which arrangement is most suitable for the uniform packing of atoms in a layer?

In crystalline silicon,$Si$ atoms occupy all the $ccp$ sites and every alternate tetrahedral void. The value of packing efficiency is closest to $....\%$