Poor Dog, Inc. borrowed $135,000 from the bank today. They must repay this money over the next six years by making monthly payments of $2,215.10. What is the interest rate on the loan?

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The interest rate can be found by using the formula for the monthly payment (PMT) of an annuity: PMT = P[r(1 + r)^n] / [(1 + r)^n - 1], where P is the principal amount ($135,000), PMT is the monthly payment ($2,215.10), n is the total number of payments (6 years * 12 months = 72 payments), and r is the monthly interest rate. Solving this equation for r requires either iterative methods or a financial calculator. Plugging the given values into a financial calculator or an appropriate financial function in a spreadsheet yields an interest rate close to 5.65% annually.