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

In a crystalline compound,$A$ atoms occupy all corners,$B$ atoms occupy all face-centered positions,$C$ atoms occupy all edge centers,and $D$ atoms occupy all tetrahedral voids formed by $A$ and $B$ atoms. Find the formula of the compound.

In an ionic solid,anions $(B)$ are arranged in a $ccp$ array and cations $(A)$ occupy $1/3$ of the tetrahedral voids. What is the formula of the ionic compound?

What fraction of one edge-centred octahedral void lies in one unit cell of $fcc$?

$A$ compound is formed from elements $X$ and $Y$. The atoms of $Y$ (anions) form a $ccp$ lattice. The atoms of $X$ (cations) occupy half of the octahedral voids and half of tetrahedral voids. What is the formula of the compound?

Percentages of free space in cubic close packed structure and in body centered packed structure are respectively