inverse of the number of years required to have the net benefits of the investment pay off the investment where the net benefits are calculated net of paying amortization "costs" of the investment. Also, different books sometimes use different letters for this formula. Fuel/power costs In the early 1970s, the cost of providing mechanical energy to move trains (locomotive operating costs) amounted to 40–43% of the total operating cost of the railways. This includes the cost of fuel/electric-power, operating/maintaining locomotives (including crew wages), maintaining the electric power system (for electrified lines), and depreciation. Of the cost of providing this mechanical energy (locomotive operating costs), fuel and power costs amounted to 40–45%. Thus fuel/power costs are very significant cost components and electric traction generally uses less energy (see #Energy-efficiency). One may plot fuel cost per year as a function of traffic flow (in net tonnes/year in one direction) for various assumptions (of ruling grades, locomotive model, single or double track, and fuel/power prices), resulting· in a large number of such plotted curves. For early 1970s energy prices of 1.3 kopecks/kwh and 70 roubles/tonne for diesel fuel, these curves (or tables based on them) show the fuel/power costs to be very roughly 1.5 to 2 time higher for diesel operation as for electric. The exact ratio, of course, depends on the various assumptions and in extreme cases of low diesel fuel prices (45 roubles/tonne) and high electricity cost (1.5 kopecks/kwh), diesel fuel costs of rail movement are lower than electricity costs. All of these curves show the difference in energy cost (of diesel vs. electric) increases with traffic flow. One may approximate the above-mentioned curves by cubic functions of the traffic flow (in net tonnes/year) with the coefficients being linear functions of fuel/power prices. In mathematics, such coefficients are usually shown as constants, but here they are also mathematical functions Such use of mathematical formulas facilitates computerized evaluation of alternatives. Non-fuel/power costs In a sense, these are components of the costs of mechanical energy delivered to the wheels of the locomotive but they are neither liquid fuel nor electricity. While electric traction usually saves on fuel/power costs, what about the other cost comparisons? Of the costs of locomotive operation, the maintenance and repair costs for electric locomotives amounted to about 6% as compared to 11% for diesel locomotives. Besides lower maintenance/repair costs it's claimed that the labor (crew) cost of operating electric