4. Suppose you borrowed a 30year mortgage of $300,000 ten years ago at the xed interest rate 6%. The interest rate is compounded monthly, and the mortgage payment is at the end of each month. (a) What is the remaining principal now? (5pt) (b) Suppose the interest rate is still 6% today. How much interest you can save if you decides to pay back the loan in the next 10 years. (519$) You may nd the following numerical results useful: (1 + 6%/12)120 = 0.549633