Explanation:
Larger sphere volume - smaller sphere volume = volume of iron in object
volume * density = mass
Volume of a sphere is formula : 4/3 pi r^3
4/3 pi ( 9.2^3 - 4.8^3) = 779 cm^3 ( NOTE: Use RADIUS not DIAMETER)
779 cm^3 * 7.9 g/cm^3 = 6154 grams = 6.15 kg
this would be for an ENTIRE sphere
a HEMI-sphere will be 1/2 of this = 3.08 kg ( or 3 kg rounded)