What is the minimum limiting radius ratio for an octahedral arrangement?

The unit cell of a $NaCl$ lattice:

For an ionic crystal of the type $AB$,the value of the limiting radius ratio is $0.40$. This value suggests that the crystal structure should be:

$A$ solid $AB$ has a $NaCl$ structure. $B$ atoms are at the corners and all face-centered positions of the unit cell,while $A$ atoms occupy all the $O.H.V.$ (Octahedral Voids). If all the atoms from the face-centered positions along one axis are removed,what will be the resulting stoichiometry of the solid?

$KBr$ crystallizes in a $fcc$ lattice (similar to $NaCl$). The second nearest neighbor of $K^+$ ion and its number are:

The coordination number of an atom in a hexagonal close-packed $(hcp)$ structure is ...........