A mathematical model extending work by Gordon and Harms is developed to describe the fissile fuel and tritium flows in a fusion-fission system consisting of a fusion hybrid reactor, a tritium production reactor, and several fission power reactors. The hybrid reactor plays the role of a fuel factory, providing the fission reactors and the tritium production reactor with their fissile fuel needs. The tritium production reactor (a fission reactor) is devoted primarily to producing tritium for subsequent use in the hybrid. Different combinations of these systems are found by shifting the tritium breeding function among the various parts. At steady state, the total thermal power in fission reactors per unit of fusion power depends only on the total conversion ratio of the fission reactors and the hybrid. An economic analysis is required to determine which combination of systems will produce electricity at the lowest cost.