Hello World,



The information I am looking for is a math expression.



If you would like to offer some additional explanations and comments and so
on, that would be fine too, but remember, all I'm looking for is a math
expression.



I need a math expression, that can be used in Excel, to calculate the
principal portion of a monthly payment, in a specific month, after a large
one-time extra payment is applied the previous month.



Let's look at an example.



Question:

What is the mathematical calculation (in Excel) that results in the
number $418.15.



Conditions:

Loan amount is $260,000

Rate is 5.625%

Loan term is 360 months

One extra payment of $25,000 is made at month number 17.

State is California.



When I use a webpage calculator I get this amortization:

The amount of monthly payment applied to principal in month 16 is 298.16.

The amount of monthly payment applied to principal in month 17 is 299.56.

An extra payment towards principal is made in month 17 of $25,000.

The amount of monthly payment applied to principal in month 18 is 418.15
(instead of 300.96).



As an example of the type of nomenclature I am looking for, the Excel
calculation that results in month 17 $299.56 is:

PPMT(5.625%, 17, 360, 260000)

equals 299.56 (negated).



My problem is that I cannot formulate an expression that accurately results
in the month 18 principal of 418.15.



I predicted the "right answer" using this web page:

http://www.decisionaide.com/mpcalcul...aPayments1.asp



According to the web page, this reduces the months by 70, from 360 to 290. I
don't know how to calculation the 290 either, so if you can help me with
that too that would be great.



Thanks World!