In many situations, the income stream to be discounted may be constant for several years. For instance, a new maintenance garage might reduce costs by $20000 per year for twenty-five years, and that cost saving is to be compared with the construction cost the garage. The flow in each year could be discounted back to the present: a quicker approach entails use of an annuity formula to compute the present value of the income stream in a single computation. If S equals the amount of the annual flow and other variables are as previously defined,