What is the packing efficiency of silver metal in its unit cell (in $\%$)?

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

Atom $X$ occupies the $fcc$ lattice sites as well as alternate tetrahedral voids of the same lattice. The packing efficiency (in $\%$) of the resultant solid is closest to

$A$ 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?

In a crystal lattice,anions $A$ form an $hcp$ array,and $2/3$ of the tetrahedral voids are occupied by cations $C$. What is the formula of the crystal?

In a compound,atoms of element $Y$ form a $ccp$ lattice and atoms of element $X$ occupy $2/3$ of the tetrahedral voids. What is the formula of the compound?