Diana usually uses a four-year payback period to determine if a project is acceptable. A recent project with uniform yearly savings over a seven-year life had a payback period of almost exactly four years, so Diana decided to find the project's present worth to help determine if the project was truly justifiable. However, the calculation did not help either since the present worth was exactly zero. What interest rate was Diana using to calculate the present worth? The project has no salvage value at the end of its seven-year life. Use the linear interpolation method when i = 0.15 and 0.20 The interest rate Diana was using is percent. (Round to one decimal place as needed.) The Biltmore Garage has lights in places that are difficult to reach. Management estimates that it costs about $2 to change a bulb. Standard 100-watt bulbs with an expected life of 1000 hours are now used. Standard bulbs cost $1. A long-life bulb that requires 90 watts for the same effective level of light is available. Long-life bulbs cost $3. The bulbs that are difficult to reach are in use for about 500 hours a month. Electricity costs $0.08/kilowatt-hour payable at the end of each month. Biltmore uses a 12 percent MARR (1 percent per month) for projects involving supplies. a. What minimum life for the long-life bulb would make it the lower cost option? b. If the cost of changing bulbs is ignored, what is the minimum life for the long-life bulb to be the lower cost alternative? c. If the solutions are obtained by linear interpolation of the capital recovery factor, will the approximations understate or overstate the required life