A bank account supervisory system for determining the amount of interest due on a shared appreciation mortgage account relating to a mortgaged property. The system comprising an input port to receive first data concerning real estate values. A computer processor being programmed to select and store in computer memory, for each account, historical and up to date data. The computer processor also being programmed to receive second data concerning loan rates. The computer processor rther being programmed to receive third data concerning rental returns. Then the system may operate to periodically calculate the value of an interest amount for both the variable and fixed loans taking into account the received data and a complex formula. Lender indicates areas for investment I n vesto rs n provide funds no approvz 2O0 approval I' given Customer enters main contract to purchase property Customer borrows 1st loan - Part A 2nd loan - Part B fixed principal + interest rate related to interest rental return Residential purchase contract obliges lender to purchase interest in the property pre-sale I Mortgage secures loans pre-sale Lenders purchase interest from customer I Joint sale of property Customer repays loans Investors Fig. 1 receive capital