A facility decides it would like to upgrade a machine. The new machine has an initial cost of C, which will be borrowed from a local bank with an annual interest rate of i_B . The old machine can be salvaged for S, which will occur immediately. The new machine will require fuel at an annual cost of f_N, and the old machine's annual fuel cost was f_O. The annual inflation rate is i_i .

Write an equation using the given variables that will determine the payback period for the upgraded machine if monthly payments are made.