$A$ compound is formed by elements $P$ and $Q$. Atoms of $Q$ are in $ccp$ arrangement. If $P$ occupies all the tetrahedral voids,what is the formula of the compound?

What is the percentage of unoccupied volume in $BCC$ structure (in $\%$)?

An alloy of metals $A$,$B$,and $C$ is found to have $A$ constituting a $ccp$ lattice. If $B$ atoms occupy the edge centers and $C$ is present at the body center,the formula of the alloy is:

Packing fraction of a layer in $2D$ hexagonal close packing arrangement is:

$A$ compound is formed by elements $A$ and $B$. Element $B$ forms an $HCP$ lattice,and element $A$ occupies $2/3$ of the tetrahedral voids. What is the formula of the compound formed by $A$ and $B$?

$A$ solid is made of two elements $X$ and $Z$. The atoms $Z$ are in $CCP$ arrangement while the atoms $X$ occupy all the tetrahedral sites. What is the formula of the compound?