I need to calculate the remaining balance on a loan given:
- fixed starting value
- fixed interest rate over the life of the loan
- size of payment can vary
- date of payment can vary (typically once/month, could be several in a
month, might be a month with no payments)

Basically given the starting balance and interest rate I just want to
record the payments as they come in.

Financial calculations (especially in Excel) aren't my specialty so any
help is appreciated.