You ae going to form a 4-tranche CMO from a pool of mortgages. The pool contains $350 million in 3-year mortgages. These mortgages make semi-annual payments. The rate on the mortgages is 4%. Treat this as an APR with semi-annual compounding. The A-Tranche holds 20% of the pool. The B-Tranche holds 20% of the pool. The C and D Tranches split the remaining portion of the pool evenly. The rates on the tranches are the rate on the underlying mortgages.

At month 18, there is a 25% prepayment.

The new amortizing payment after the pre-payment (in dollars) is