A large lithium-ion phosphate battery pack for an indus trial application is expected to save $20,000 in annual e nergy expenses starting at year two from its 7-year life. For a 3-year simple payback period, the permissible ca pital investment is $60,000. What is the internal rate of return on this $60,000 battery pack if it has a residual v alue of $10,000 at the end of 7 years? The MARR is 18% per year