Please help me understand how to solve this step by step. Thanks in advance.

Chapter Nine Risk Management: Asset-Backed Securities, Loan Sales, Credit Standbys, and Credit Derivatives b. Every month principal and interest are paid on the outstanding mortgages, and some mortgages are paid in full. These payments are passed through to Pretty Lake, and the trustee uses the funds to pay coupons to CMO bondholders. What are the coupon payments owed for each tranche for the first month c. If scheduled mortgage payments and early prepayments bring in $5 million, how much will be used to retire the principal of CMO bondholders and which tranche will be affected? d. Why does Tranche D have a higher expected return