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If I borrow \$15,000.00, and the interest rate is 18%, amortized over 20 years. What amount of interest will I pay?

I say it's \$5,400.00, she says it's \$54,000.00. Please help us with this, it's just getting silly, and no it's not homework ,lol. It is derived from someone else's question. Than-you :P

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When calculating interest rate, you use the formula I = PRT where P is principle, R is rate and T is time. 15,000 x 0.18 x 20 = \$54,000. Check out the link below for additional details http://math.about.com/od/businessmath/ss/Interest.htm.

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Okay thank JJ, but I'm still confused, I don't understand how an \$15,000.00 loan @ 18% will equal \$54,000.00, that's nuts. I always believed you moved the decimal point one to the left for a total of \$5400.00. Your link seemed to tell me the same. In addition, I went to a loan interest calculator site. It read if a loan was made in the amount of \$10,000.00 at the rate of 6% in one year the interest is 600.00, so in 10 years, thats \$6,000.00. What am I missing here? Thanks again :)
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20 years of accumulated interest is a lot of time
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The problem here is the very long period of time. You are making 240 very small payments on principal, so the interest just keeps accumulating month after month.
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Gotcha, thanks!
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Assuming that the interest is compounded monthly, my Excel PMT computation gives a total payout of \$55,559.21, so the total interest paid is \$40,559.21, thus demonstrating the error of borrowing small sums over large periods of time.