(17B/19B) GPM: initial payment

[Werner]

To perhaps better see what I did to solve the problem on the 12C, here's an equivalent Plus42 equation, using its built-in TVM functions:

G42: PV-PMT×FV(NG:PV(NP:I%:I%:-G%:1:0):USPV(I%,N-NP×NG):USPV(I%,NP):1:0)

The PV() part calculates K%, the effective rate of applying the growth factor G% and dividing by (1+I%/100)^NP combined;
The FV() part then calculates the Present Value (yes.. even if it's FV) of a PMT of 1.00

Cheers, Werner