A=P((1-(1+r)^(-n))/r, where A is the present value of the annuity, or the amount of the loan, P is the periodic payment, r is the interest rate per period, and n is the number of periods. In this case, I assume the payments are monthly, so n would be 36. You mentioned that you already have A and P. However, solving for r algebraically is not that easy because it is in two places on the right hand side. However, you can make a table and put in interest rates to make both sides equal. Remember to multiply this answer by 12 to get the annual interest rate.
For example, if the payment is $100, and the amount of the loan is $2,766.07, and the number of periods is 36, what is the interest rate.
r Calculate Actual Difference
0.010 3010.75 2766.07 -244.68
0.011 2959.42 2766.07 -193.35
0.012 2909.33 2766.07 -143.26
0.013 2860.42 2766.07 -94.35
0.014 2812.68 2766.07 -46.61
0.015 2766.07 2766.07 0.00
0.016 2720.55 2766.07 45.52
0.017 2676.11 2766.07 89.96
You can see from the chart that the value of r of 0.015 makes the difference 0, so the periodic interest rate is 0.015 or 1.5%. We need to annualize this by multiplying by 12 and we get an annual interest rate of 18%.
Good luck,
Paul
Annuity Payment PV
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