interest amount on partial payment

[Linda410]

Hello,
I appreciate any help I get,
I need to figure out the interest amount on partial payment, for example
If loan amount is $7080.82 with monthly payment $325. This first payment made full $325.00, subtracted $118.25 as interest, to principle is $206.75. no issue here

My question is: what if the payment just $100 next month, while the interest amount will be $114.8

Is this mean my to principle will be -$4.8 or else.
How can I calculate the interest on that $100.00 with new balance ($7080.82-$118.25)= $6874.07 after my above first payment?

Thank you for any help

 

[Linda410]

First thank you for your time and reply,
Anyway, you got the list correct,
simply I need to find out what will be the interest if payment less than interest as your schedule ( simple interest on monthly payment).
BTW: Not sure what is the wrong with my English, sorry if wasn’t clear on my question .
I know this is math question, I appreciate you tell me what is the wrong with my English.