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April 18th, 2011, 09:11 AM  #1 
Newbie Joined: Apr 2011 Posts: 2 Thanks: 0  Calculate interest on loan
Can anyone offer me an equation or direct me to where I might find an equation that could find the interest rate on a loan with only the following information: Initial loan amount, current loan amount, number of payments made, monthly payment amount. I've found equations that are useful if you know the financed term but I don't have access to that information. Any help would be greatly appreciated!

April 18th, 2011, 09:39 AM  #2 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms  Re: Calculate interest on loan
You could set it up in Excel and use goal seek. You can get an equation in closed form (as a geometric series) but it will be messy. 
April 28th, 2011, 08:03 AM  #3 
Newbie Joined: Apr 2011 Posts: 2 Thanks: 0  Re: Calculate interest on loan
I have no math knowledge and wouldn't even know where to begin to come up with a formula like that, I understand it would be messy but does anyone know of an equation or know where I could find one that would solve this? I have been looking for awhile and haven't found anything...

April 28th, 2011, 11:17 AM  #4 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms  Re: Calculate interest on loan
I don't know of a source that would write something like that out in full. It's like the general solution for the quartic  it can be written out but no one does. If it's important enough to you I'm sure you could pay someone to write a spreadsheet (it might take them two hours). If an approximation is good enough you can use the equations you have to work forward with different guesses as to the interest rate until one works out to be close to what you want. 

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