有谁知道mortgage securities 是怎么计算回报的

博迪的投资学上有：

surpose that ten 30-year mortgages,each with a principal value of $100,000 , are grouped together into a million-dollar pool.if the mortgage rate is 10% , then the firist month's payment for each loan woule be $877.57,of which $833.33 would be interest and $44.24 would be principal rapayment.