Different formulas one cell! HELP!

I have a budget sheet that I have been working on. Everything is great but now I want to add my credit card APR so that I see the actual pay down.

Sheet is set up now as BALANCE | Monthly payments (I have it set that I can potentially make a payment every Friday if I wanted to, so 4-5 possible cells to subtract) | New Balance. This balance is then carried over to then next sheet for the next month. What I would like to do now is after I subtract my payments is to multiply the new balance X the APR percentage / 4. To make this easier I took my APR /4 then made that a decimal and put that into a cell. But formula doesn't work. What am I doing wrong?

EXAMPLE:
DUE DATE BALANCE PAYMENT NEW BALANCE
Credit Card 1st $ 1000 $ 100 $ 900

WHAT I WOULD LIKE:

DUE DATE BALANCE PAYMENT NEW BALANCE
Intrest Rate Credit Card 1st $ 1000 $ 100 $ 920

Thank you in advance!

Can you post a sample file csacord?