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

In $NaCl$ type structure, the shortest distance between two nearest neighbours is $100 \ pm$. Then, the distance between two next nearest neighbours in the same unit cell will be:

An element occurring in the $bcc$ structure has $12.08 \times 10^{23}$ unit cells. The total number of atoms of the element in these cells will be:

In the zinc blende structure,zinc atoms fill up:

$A$ solid $A^{+}B^{-}$ has a $NaCl$ type close-packed structure. If the radius of the anion is $250 \ pm$, what should be the ideal radius of the cation in $pm$?

In a mineral $MX_2$,$M^{+2}$ ions are arranged in a $ccp$ structure and $X^{-}$ ions occupy the tetrahedral voids. What are the number of cations and anions per unit cell,the coordination number of the cation,and the percentage of occupied tetrahedral voids,respectively?