A neutrino beam with E = 143 GeV is passed through a slab of aluminum-27 (with 27

Question:

A neutrino beam with E = 143 GeV is passed through a slab of aluminum-27 (with 27 nucleons in each nucleus). The probability that a neutrino in the beam will scatter off a nucleon in the aluminum slab is 4.19 ∙ 10-12. The scattering cross section is given by σ(E) = (0.68 ∙10-38 cm2 GeV-1)E, and aluminum has a density of 2.77 g/cm3. How thick is the slab?