What is the number of spheres in contact with a tetrahedral void?

In a face-centred cubic lattice,atom $A$ occupies the corner positions and atom $B$ occupies the face-centred positions. If one atom of $B$ is missing from one of the face-centred points,the formula of the compound is

What is the percentage of space occupied by spheres of equal size in $hcp$ and $ccp$ three-dimensional arrangements?

Explain the calculation of packing efficiency in $hcp$ and $ccp$ structures with the help of a diagram.

The ratio of close-packed atoms to tetrahedral holes in cubic close packing is

$A$ compound made up of elements $A$ and $B$ (with a general formula $A_x B_y$), where $B$ forms an $hcp$ lattice and $A$ occupies $2/3$ of the tetrahedral voids. The formula of the compound is