First of all, I don't think the question was clear enough. Therefore, I had to assume they are referring to the volume of the unit cell.
V=a^3, side length a
aBCC=2R√2, aFCC=4√3/3R
%change=(VFCC-VBCC)/VBCC
I thought this was right until I checked with others who did this:
so the only...