I have a credit card and I'm trying to calculate future interest charges per month. I have 4 months of statements that I have had to reverse engineer their math for. It took me a few days, but I finally figured out this banks algorithms for calculating their interest (I tried to get their help, but nobody could figure it out lol... yeah...).
However I am stuck on one part I can't figure out. They break apart the payment into proportional parts. According to them, they're putting the majority towards the soonest promotional end date is all I got from them. It seems that each charge/balance has it's own weight to it.
So it's deferred interest, so the payment amount is not tied to any interest\interest calculations. Just focusing on one month, we have the following:
$$ \text{Payment Amount:}\quad \text{\$25.00}\quad\qquad\qquad \qquad\qquad\qquad \qquad\qquad$$
$$ \text{Before payment:} \quad \text{\$975,}\quad \quad \; \text{\$800,} \qquad\text{\$17.98,} \qquad \text{\$74.99} $$
$$ \text{After payment:}\; \quad \text{\$961.94,} \quad \text{\$789.30,}\quad\text{\$17.74,} \qquad \text{\$73.99} $$
$$ \text{Difference of:}\; \qquad \text{\$13.06,} \quad \text{\$10.70,}\qquad\text{\$0.24,} \qquad \text{\$1.00} $$
The $975's deferment date ends 9/15/2018. All the rest end on 10/16/2018. I can't even think of where to start with this one, maybe my mind is zapped from figuring out their crazy system for their interest, but any help is greatly appreciated.
Thanks!
(Probably not needed for this calculation, but this months statement is from 10/17/2018 to the end of 11/15/2018, with the payment on 11/08/2018.)
They've paid off the same proportion (rounded to the nearest cent) of each. Specifically, for a total payment of $p$, and amounts owing $x_1, \ldots, x_n$, they've subtracted $$\frac{px_k}{\sum\limits_{i=0}^nx_i}$$ from each (rounding to the nearest penny, possibly with some check in there to make sure that the rounding all cancels out). Whatever they say about "putting the majority towards the soonest promotional end date", they aren't. Feel free to check the other months to be sure that this pattern holds up.