7. An annuity has payments at the end of every month for 40 years. For the first year, the monthly payments are $100 each. For each subsequent year, the payments increase by $20, so the payments in the second year are all $120, the payments in the third year are all $140, etc. The nominal interest rate is 9% compounded monthly. Find the accumulated value of this annuity at the time of the last payment.