You have an outstanding student loan with required payments of $ $500$ per month for the next four years. The interest rate on the loan is $9\%$ APR$($monthly$).$ You are considering making an extra payment of $ $100$ today$($that is$,$ you will pay an extra $ $100$ that you are not required to$$ pay$).$ If you are required to continue to make payments of $ $500$ per month until the loan is paid$$ off, what is the amount of your final$$ payment? What effective rate of return$($expressed as an APR with monthly$$ compounding$)$ have you earned on the $ $100?$