What does a tetrahedral void in a crystal represent?

For the given close-packed structure of a salt made of cation $X$ and anion $Y$ shown below (ions of only one face are shown for clarity),the packing fraction is approximately
$\text{(packing fraction} = \frac{\text{Packing efficiency}}{100}\text{)}$

What is the total number of spheres that surround the tetrahedral hole in a close-packed three-dimensional structure?

$A$ compound is formed by two elements $M$ and $N$. The element $N$ forms $ccp$ lattice and atoms of $M$ occupy $1/3$ of the tetrahedral voids. What is the formula of the compound?

In a $ccp$ structure of a mixed oxide,$1/8$ of the tetrahedral voids are occupied by divalent $X^{+2}$ ions and $50\%$ of the octahedral voids are occupied by trivalent $Y^{+3}$ ions. The formula of the oxide is:

$A$ 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?