In an $FCC$ unit cell,what fraction of the edge length is not covered by atoms?

$A$ 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 $.....$

If the unit cell of a mineral has cubic close packed $(ccp)$ array of oxygen atoms with $m$ fraction of octahedral holes occupied by aluminium ions and $n$ fraction of tetrahedral holes occupied by magnesium ions,$m$ and $n$,respectively,are

How will you distinguish between the following pairs of terms:
$(i)$ Hexagonal close-packing $(hcp)$ and cubic close-packing $(ccp)$?

Write a short note on: Formula of a compound and number of voids filled.

In an ionic solid compound,$A$ ions are at the corners of the cube and $B$ ions are at the center of each face. The molecular formula of the compound will be ..............