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

What is the percentage of empty space in a $bcc$ unit cell (in $\%$)?

Atoms $A$ are arranged in an $FCC$ system,and atoms $B$ occupy all the octahedral voids and half of the tetrahedral voids. The formula of the compound is:

Atoms of element $B$ form $hcp$ lattice and those of the element $A$ occupy $2/3$ of tetrahedral voids. What is the formula of the compound formed by the elements $A$ and $B$?

Vacant space in body centered cubic $(BCC)$ lattice unit cell is about (in $\%$)

$A$ face-centred cubic $(FCC)$ unit cell is made up of two types of atoms $A$ and $B$,in which $A$ occupies the corner positions and $B$ occupies the face centres. If atoms along one of the body diagonals are removed,what is the empirical formula of the remaining solid?