"Stonem248" wrote:
I need help on a formula to determine the amount paid as
interest on a loan if you pay $200.00 extra each month.

I assume you mean, for example, that if you have 15-year
loan of $100,000 at 5% compounded monthly, resulting in a
monthly payment of $790.79, what would the cumulative
interest be if you paid $990.79 per month instead?

I do not find an Excel function that would compute that, since
CUMIPMT() does take the monthly payment as an argument,
as NPER() does :-(.

The approximate cumulative interest would be:

=990.79*NPER(5%/12, 990.79, -100000) - 100000

That calculates to $30,021.86. In an annuity table, the actual
cumulative interest is $30,022.22.

Close enough for government work, although you might miss
Pluto :-).