$A$ 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 $B$?

Which of the following is correct for a two-dimensional closed-packed crystal?

The fraction of voids occupied in the inverse spinel compounds are

$A$ 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?

In a metal oxide,$O^{2-}$ ions are arranged in $hcp$ lattice while metal ions occupy $2/3$ of the octahedral voids. The simplest formula of the oxide is:

$A$ 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 $.....$