What is the minimum number of spheres required to develop a tetrahedral void?

In an ionic solid,anions $(B)$ are arranged in an $hcp$ array and cations $(A)$ occupy $\frac{2}{3}$ of the octahedral voids. What is the formula of the ionic compound?

$A$ compound of formula $A_2B_3$ has the $hcp$ lattice. Which atom forms the $hcp$ lattice and what fraction of tetrahedral voids is occupied by the other atoms?

In a metal oxide,$O^{2-}$ ions are arranged in $hcp$ lattice while metal ions occupy $2/3$ of the octahedral voids. The simplest formula of the oxide is:

For a simple cubic $(SC)$,body-centered cubic $(BCC)$,and face-centered cubic $(FCC)$ unit cell with edge length $a$,what is the ratio of the radii of the atoms present in them?

In a compound $XY$,the atoms of element $Y$ form a $ccp$ structure,while the atoms of element $X$ occupy $2/3$ of the tetrahedral voids. The molecular formula of the compound will be ..........