What is the value of the radius ratio for an ionic crystal having a coordination number of $6$?

The structure of $Na_2O$ crystal is

The radius of the calcium ion is $94 \text{ pm}$ and the radius of the oxide ion is $146 \text{ pm}$. What is the possible crystal structure of calcium oxide?

$A$ compound $M_pX_q$ has a cubic close packing $(ccp)$ arrangement of $X$. Its unit cell structure is shown below. The empirical formula of the compound is:
$M = \square$
$X = \bigcirc$

If $AB$ crystallizes in a zinc blende structure with $B^{-}$ ions at lattice points,what fraction of tetrahedral voids is occupied by $A^{+}$ ions? (Express as a percentage)

In a $ccp$ lattice of $X$ and $Y$,$X$ atoms are present at the corners while $Y$ atoms are at face centers. What would be the formula of the compound if one of the $X$ atoms from a corner is replaced by a $Z$ atom?