Given 310 Pre-PMT assumption, the current Pre-PMT rate of the pool is closest to a CPR of:
So w (4 months / 30 ) x .06 x 3.1 = .0248 or 2.5%.
However, when the book explains the concept (Mortgage Passthrough Securities 3.D.4), its example says the WAM is 357, meaning loans are seasoned 3 months. There’s a chart but it says, “Therefore the CPR used is the CPR that corresponds to 4 months. From the PSA, the CPR is 0.8% (4 x 0.2%).
That seems to directly contradict the way it’s done in the mock. If you used 5 months in the problem, you’d get 3.1%.