calculating marginal profit Announced on the UK economy in the international monetary fund annual health check results, published in the Treasury's speech, ms lagarde hit back at the recession after the referendum criticism at the upcoming future fund. The investment quantity is the decreasing function of interest rate, saving is the increment function of interest rate, and the change of interest rate depends on the equilibrium point of investment quantity and saving amount. Keynes's theory of money supply and demand decided that interest rates were a monetary factor rather than an actual factor. The accounting income maintains the financial capital preservation, namely, the monetary value that only requires the owner to invest is not eroded, the enterprise income exceeds the input value part is the accounting income. Accounting focuses on reporting financial results. The economic benefits insist on physical capital preservation, which can only be confirmed when the actual production capacity of the owner is saved. It is believed that only by maintaining the actual production capacity of the enterprise can the enterprise reproduce smoothly. Physical capital preservation is more meaningful than the income of financial capital preservation, but it is difficult to measure.