Suppose an investor with a 7-year investment horizon is considering the purchase of a 4.50% APR, monthly payment, mortgage with 22 years (264 months) remaining until maturity. The mortgage currently has an outstanding balance of $245,000 and is selling to offer a YTM of 4.8% on the secondary market. The investor expects to be able to reinvest the first 36 monthly cashflows at 4.8% (over their entire reinvestment interval), but expects to be able to reinvest the last 48 monthly payments at only 4.5%. At the end of her investment horizon, she expects to be able to sell the mortgage at a YTM of 4.5%. What is the total/expected (effective) return offered by this security?

Please explain