What is the coordination number of a particle in $hcp$ structure?

For $bcc$ cubic coordination the value of radius ratio is

$A$ compound made of elements $A$ and $B$ forms an $fcc$ structure. Atoms of $A$ are at the corners and atoms of $B$ are present at the centres of the faces of the cube. What is the formula of the compound?

The crystal structure of the solid compound $XY$ is similar to $NaCl$. If the radius of the cation $X^+$ is $100 \, pm$, then the radius of the anion $Y^-$ will be ......... $pm$.

In the zinc blende structure,zinc atoms fill up:

If the length of the body diagonal of a cube is $x$,then the tetrahedral void will be at a distance of ....... from the center of the cube.