The Browns wish to accumulate at least $150,000 at the time of their last deposit in a college fund for their daughter by contributing an amount A into the account at the end of each year for eighteen years. What is the smallest annual payment A that will suffice if the college fund earns a level annual effective interest rate of 5%? If at the end of ten years, it is announced that the annual effective interest rate will drop to 4.5%, how much must the Browns increase their payments in order to reach their accumulation goal? Assume that the Browns wish to continue to make level payments except for a slightly reduced final payment.

Answer: Based on the 5% rate, the first 17 payments should each be for $5,331.94, and the last payment should be $5,331.75. If the rate changes to 4.5% immediately after the tenth payment, the next 7 payments should be for $5,823.82, a reduction of $491.88, and the new final payment should be for $491.81.

I just don't know how to go about getting the answer using the formula.