The perpetuity part is really confusing. Not sure how to represent it on part b

Exercise 2: Long Term Discounting Assume a project will result in benefits of $10 million dollars each year in perpetuity. (a) Compute the present value of these benefits using a time-constant discount rate of 3%. (b) Compute the present value of these benefits using the following time-declining discount rate schedule: 4 percent for years 1-50; 3 percent for years, 51-100; 2 percent for years 101-200; 1 percent for years 201-300; and 0.5 percent thereafter. (c) Between which values should the discount rate in part (a) fall in order for both NPV (in (a) and (b) ) to not be more than a 10% apart from each other