What is the minimum number of spheres required to develop a tetrahedral void?

Identify the types of voids represented by the structures $I$,$II$,and $III$ in the given image.

In a face-centred cubic arrangement of $A$ and $B$ atoms,$A$ atoms are at the corners of the unit cell and $B$ atoms are at the face centres. If three of the $B$ atoms are missing from the face centres in the unit cell,what is the simplest formula of the compound?

In a compound $XY$,the atoms of element $Y$ form an $fcc$ structure,while the atoms of element $X$ occupy $25\%$ of the tetrahedral voids. The molecular formula of the compound will be .......

What is the packing efficiency of the two-dimensional square unit cell shown below (in percentage) (in $\%$)?

$A$ 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?