$${B = - \frac{\mu_0 c}{4r} (r^4 - a^4) \\ = \frac{(4 \pi \times 10^{-7}\, \mathrm{T \cdot m/A})(3,0 \times 10^6\, \mathrm{A/m^4})}{4 (0,030\, \mathrm{m})} \\ \times [(0,030\, \mathrm{m})^4 - (0,020\, \mathrm{m})^4] \\ = - 2,0 \times 10^{-5}\, \mathrm{T}}$$