Errata ....

On Nov 18, 11:45 am, I wrote:
Note: You said "2 payments in advance". That's ambiguous. If the
normal payment is $1000, which do you mean you paid in advance:
a total of $2000 or $3000?

The first $1000 is normally considered payment of both interest and
principal as if paid on the due date.

Although generally true for interstitial "advance payments", this
probably does not apply to the case you are talking about. All of any
amount paid at the outset of a loan reduces the principal.

What I was trying to say before is: you cannot compute payment based
on the __initial__ loan such that it reduces the __initial__ loan to
zero in n periods, and it reduces the __reduced__ loan to zero in the
same n periods, where the reduced loan is the initial loan less twice
that payment.

However, you __can__ compute a payment that reduces (only) the
__reduced__ loan to zero in n periods such that the reduced loan
amount is the initial loan less twice that payment. That's probably
what you want.

I doubt that you can compute that payment using PMT. However, you can
use the following formula to compute the payment:

=-pv*(1+rt)^n * rt / ( (1+2*rt)*(1+rt)^n - 1 )

where rt = i/n, i = annual interest rate, pv = initial loan amount
(positive number), and n = term of the loan.

Note that that computes payment as a negative number. If you want a
positive number, change "-pv..." to "pv..." (eliminate the unary
minus).

BTW, the factor "2*rt" can be replaced by "k*rt", were k = number of
advance payments.

HTH.