[Vishal]

Hi friends
My question is as follows:
If edge length of unit cell of sodium chloride is 600 pm, and the ionic radius of chlorine ion is 190 pm, the the ionic radius of sodium ion is
a)310 pm    b)110 pm c)220 pm d)none of these

The confusion created is the ratio of sodium ion to chloride ion in NaCl structure.
Ratio of sodium ion to chloride ion is 0.525 but in rock salt it is 0.563
Which should be selected as the ratio? 

[AWK]

Just sketch structure or find it. Solution is very simple.
And be careful. Question concerns radius not diameter!