(17B/19B) GPM: initial payment

[Werner]

Indeed. What the article fails to mention though, is what should be done if you want both Graduated Payments and have a minimum payment equal to the interest due?
The above equations can be used to solve for any of the other parameters, so if the initial PMT is set, we can vary another one, like G% or N (the number of times the payment grows)
For the problem with
M = 30 years
PV = 60'000
I%Y = 12 per year
N=5 (growth periods)
we set
PMT = -PV*I%/12 = -600

Solve for G% to see 0.80% (0.8053452), so the last 300 payments are -600*(1+G%/100)^5 = -624.55

or, take instead
N=30 (let payments grow annually all through the loan - N=29 is the same BTW)
Now G%= 0.40% (0.3973548653), bringing the last 12 payments to
-600*(1+G%/100)^29 = -673.13

As a regular annuity, this mortgage has a constant monthly PMT of -617.17
(which, incidentally, you can also determine using this equation, setting N or G% to 0)

Cheers, Werner