Can you please help me solve this on a financial calculator.

question b and c are pretty straightforward in this problem. However, i cant understand how to solve for NPV when the investment lasts forever. What value do i use for n?

Exercise #2: If you insulate your office for $28,000, you will save $2,800 a year in heating expenses. These savings will last forever. A. What is the NPV of the investment when the cost of capital is 4%? NPV =5 ......... [4] B. What is the IRR of the investment? C. What is the payback period on this investment? ............ [6] .................... years