i worked the problem down to the second equation on the second page, just cant figure out how they switch

Newton3: We know that the goal of a moderator is to slow the fast fission neutrons down to thermal energies, where the cross section for fission reactions is several orders of magnitude larger. If the fraction of energy lost (that is \% energy lost) by a neutron in a collision with the moderator is constant over the entire energy range as the neutron slows down, show that the number of collisions, N, that the neutron undergoes with the moderator material is: N=ln(Efast/Ethermal)/ Want N=ln(Erest/Ethem)/ from: Efres=(EfrotETherm)N(EIntitid) =ln(EEinadEsnit+)