Monthly mortgage coupon rates should be calculated by simply dividing the annual rate by 12. You should also assume that all of the securities pay monthly. You should also divide annual interest rates by 12 to get the corresponding monthly rate and assume monthly compounding when computing present values.

(Principal-Only MBS and Interest-Only MBS)Suppose we construct principal-only (PO) and interest-only (IO) mortgage-backed securities (MBS) using the mortgage pass-through of the previous questions. Assume a prepayment multiplier of 100 PSA. What is the present value of the PO MBS if we use an annual risk-free rate of 4.5% to value the cash-flows?