Diana usually uses a three-year payback period to determine if a project is acceptable. A recent project with uniform yearly savings over a five-year life had a payback period of almost exactly three 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 five-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.)