• thickness of the hollow ball: t = 2 cm;
• radius to the outside surface: R = 6 cm;
• radius to the inside surface:
r = R – t
r = 6 – 2
r = 4 cm.
The answer you're looking for is the difference between the volume of two spheres:
• Volume of a sphere whose radius is R = 6 cm:
4π · R³
V₁ = —————
3
4 · 3.14 · 6³
V₁ ≈ ———————
3
4 · 3.14 · 216
V₁ ≈ ————————
3
2712.96
V₁ ≈ ——————
3
V₁ ≈ 904.32 cm³ (i)
• Volume of a sphere whose radius is r = 4 cm:
4π · r³
V₂ = —————
3
4 · 3.14 · 4³
V₂ ≈ ———————
3
4 · 3.14 · 64
V₂ ≈ ———————
3
803.84
V₂ ≈ —————
3
V₂ ≈ 267.95 cm³ (ii)
—————
So the volume of rubber used is
V = V₁ – V₂
V ≈ 904.32 – 267.95
V ≈ 636.37 cm³
V ≈ 636.4 cm³ <——— this is the answer (3rd option)
I hope this helps. =)