Boral is the material composed of 50 % al, 40 % b, and 10 % c by weight. its density is 2.56 g/cm3. a beam of 0.025-ev neutrons with intensity of 5´106neutrons/(cm2×s) strikes a target of 2.5 cm2 in area made of the boral.

calculate the atom densities of al, b, and c in boral.
calculate the macroscopic cross-sections sa, ss, and st for boral at the neutron energy 0.025 ev.
how thick must the boral target be in order to reduce the intensity of such neutron beam by a factor of 7 ?