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

Which one of the following statements about packing in solids is incorrect?

An ionic compound is formed by elements $X$ and $Y$. Element $Y$ forms a face-centered cubic $(FCC)$ lattice, and element $X$ occupies one-third of the tetrahedral voids. What is the formula of the compound?

With the help of a labelled diagram,show that there are $4$ octahedral voids per unit cell in a cubic close-packed $(CCP)$ structure.

$A$ compound is formed from elements $X$ and $Y$. The atoms of $Y$ (anions) form a $ccp$ lattice. The atoms of $X$ (cations) occupy half of the octahedral voids and half of tetrahedral voids. What is the formula of the compound?

$A$ two-dimensional solid is made by alternating circles with radius $a$ and $b$ such that the sides of the circles touch. The packing fraction is defined as the ratio of the area under the circles to the area under the rectangle with sides of length $x$ and $y$. The ratio $r=b/a$ for which the packing fraction is minimised is closest to $.....$