Correct Answer - (a) 195 pm (b) 128.1 pm
(a) Distance between the oppositely charged ions in the lattice `=(("390 pm"))/(2)` = 195 pm.
(b) For a cubic unit cell, `(r_(A^(+)))/(r_(B^(-)))=0.732`
`r_(A^(+))=0.732xxr_(B^(-))=0.723xx("175 pm")=128.1 "pm"`