1. **State the problem:** Calculate the interest to be paid on September 1 using the unpaid balance method. 2. **Given data:** - Previous balance $= 2596.77$ - Interest rate $= 12\%$ per year 3. **Formula used:** The unpaid balance method calculates interest based on the previous balance without considering any payments made during the billing cycle. The interest for one year is given by: $$\text{Interest} = \text{Previous Balance} \times \text{Interest Rate}$$ Since interest is usually calculated monthly, we divide the annual rate by 12: $$\text{Monthly Interest} = \text{Previous Balance} \times \frac{\text{Annual Interest Rate}}{12}$$ 4. **Calculate monthly interest:** $$\text{Monthly Interest} = 2596.77 \times \frac{12}{100} \times \frac{1}{12} = 2596.77 \times 0.01 = 25.9677$$ 5. **Interpretation:** The interest to be paid on September 1 using the unpaid balance method is approximately $25.97$. **Final answer:** $$\boxed{25.97}$$