How many unit cells share a corner of a unit cell in a crystal lattice?

$A$ crystalline solid $XY_3$ has a $ccp$ arrangement for its element $Y$. $X$ occupies:

In a face-centred cubic $(FCC)$ lattice,where are the tetrahedral voids located?

What is the packing efficiency of $fcc$ crystal structure (in $\%$)?

$A$ compound is formed by two elements $A$ and $B$. Atoms of the element $B$ (as anion) make $ccp$ lattice and those of element $A$ (as cation) occupy all tetrahedral voids. The formula of the compound is

Calculate the total number of tetrahedral and octahedral voids in $0.4 \ mol$ of a compound having such voids in it.