Question: In a certain CMBS issue, $500 million of senior securities and $100 million of mezzanine securities are issued. The coupon on the senior securities is 7%, and that on the mezzanine is 9%. The average contractual interest rate in the underlying mortgage pool is 10%. Assuming annual interest payments and no par value retired or defaulted, how much residual interest will be available for an IO tranche from these two par-valued tranches at the end of the first year?