catalogue interest and calculation(math Q)

MrPT

In an effort to try and clear some debts i am trying to pay a little extra each month to the greatest effect

What i was just trying to do was create a spreadsheet of my current catalogue balance and payments then create add a formula to show how my balance changes by increasing my monthly payments by £5, £10 etc

But try as i might i cannot (working on past and current statement) get my formula to equal the same APR or interest stated on my statement.

So can anyone tell me how to basically reverse engineer a formula using

A-Current balance
B-Minimum payment and
C-interest (not their stated rate but the amount that was added ast month)

It would actually be really nice even just to check i am actually paying the interest rate they say i am as they have changed hands (and rates) alot in the last 2 years

theoretica

You need to work in days- as interest may well be charged by the day so you will pay less in February than March. Also the exact day your payment goes through will affect things. Though they may charge interest by the month- it depends on your small print...