Which crystal lattice has the maximum packing efficiency?

$A$ compound is formed by two elements $A$ and $B$. The atoms of element $B$ form $ccp$ structure. The atoms of $A$ occupy $1/3$ of tetrahedral voids. What is the formula of the compound?

Find the number of tetrahedral voids per atom in a crystal lattice.

The stacking pattern in a body-centered cubic $(bcc)$ lattice is of the type............

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

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 ..........