1. **State the problem:** We have a backpack originally priced at 30 and it is discounted by 40%. We need to find the sale price. 2. **Formula used:** The sale price after a discount is given by: $$\text{Sale Price} = \text{Original Price} - \text{Discount Amount}$$ where $$\text{Discount Amount} = \text{Original Price} \times \frac{\text{Discount Percentage}}{100}$$ 3. **Calculate the discount amount:** $$\text{Discount Amount} = 30 \times \frac{40}{100} = 30 \times 0.4 = 12$$ 4. **Calculate the sale price:** $$\text{Sale Price} = 30 - 12 = 18$$ 5. **Explanation:** The discount reduces the original price by 40%, which is 12. Subtracting this from the original price gives the sale price of 18. **Final answer:** $$\boxed{18}$$