CFAI EOC Rdg58 ABS #15: CPR calc inconsistency

I am confused with CPR calc. Please help! Looks like a custom HELOC ramp: “The model used with respect to the loans (the “prepayment ramp”) assumes that the home equity loans prepay at the rate of 5% in the first month after origination, and an additional 1.8% each month thereafter until the 12th month. Beginning in the 12th month and each month thereafter, the prepayment ramp assumes a prepayment rate of 24.8% CPR” Calc the CPR assuming 200% PPC (same as PSA for HELOC). I find inconsistency in how the book calcs the CPR. Is it how it works or is it time for me to get some rest right now? Month 2: (5+1.8)*2 = 13.6 Month 11: [5+(1.8*11)]*2 = 49.6%

I dont have the book in front, but CPR for MBS is .2% increase for each of the first 30 months, then 6% after. Where did those numbers come from? Once you have the CPR, you can calculate SMM = 1-(1-CPR*PSA)^(1/12).