A hollow spherical iron ball has a diameter of $15\mathrm{.}3cm$ and has a mass of $12\mathrm{.}3$ kilograms. Assuming the hole inside the ball is spherical with the same center as the center of the ball, what is the thickness in cm of the layer of iron surrounding the hole? The density of iron is $7\mathrm{.}86\frac{g}{c}{m}^3.($The volume of a sphere is $(\frac{4}{3})r^3.)$