excel formula for credit card interest pulling values from diferent spreadsheet

I have spreadsheets setup that has every 1st and 15th of every month in one workbook. A cell in the current time period pulls the value of one cell and subtracts the payment amount from a different cell of the previous time period. The formula looks like this:  =('040112'!A34)-('040112'!A6)
I would like to add the monthly interest rate to this formula. How to I set that up?
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knada242Asked:
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dlmilleCommented:
You'll need to set a range (e.g., A1) equal to your Annual Interest Rate (APR%) (unless you want to hard code it).  I'm assuming A34 holds your current balance.

As a result, I would suggest the following formula enhancement for theoretical New Balance in Column B:

[B34]=('040112'!A34)*MonthlyRate/2 - ('040112'!$A$6) 'Monthly Rate hardcoded and copy down

or

[B34] = ('040112'!A34)*$A$1/24 - ('040112'!$A$6) 'and copy down

That would take your monthly rate (based on annual APR%/12), divide by two (because you're making two payments/month, so we get a balance at the mid-period) and  multiply it by the outstanding balance, then take the bi-monthly payment at range A6 out of it.  

I'm assuming that at range A6 you have a certain payment you're using to pay down the credit card bill, and thus would need $A$6 to lock in that reference, so it is subtracted for every revised balance, as you copy down.

Cheers,

Dave
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knada242Author Commented:
Thank you for your response. You got me moving in the right direction. I came up with:

=('040112'!A34)*(0.16/12)+('040112'!A34)-('040112'!$A$6)

Right now I am only calculating the interest on a monthly basis in the sheet that is right after the statement date. As time goes on I will refine it more, but this gives me a much better idea of how to build my budget.

Thank you for your help.
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