1. **State the problem:** We want to find how many liters of a 25% saline solution must be added to 3 liters of a 10% saline solution to get a 15% saline solution. 2. **Set variables:** Let $x$ be the liters of 25% saline solution to add. 3. **Write the equation for the amount of salt:** - Salt in 10% solution: $3 \times 0.10 = 0.3$ liters - Salt in 25% solution: $x \times 0.25 = 0.25x$ liters - Total salt after mixing: $0.3 + 0.25x$ 4. **Write the equation for total volume:** - Total volume after mixing: $3 + x$ liters 5. **Set up the concentration equation:** The final solution is 15% saline, so $$\frac{0.3 + 0.25x}{3 + x} = 0.15$$ 6. **Solve the equation:** Multiply both sides by $3 + x$: $$0.3 + 0.25x = 0.15(3 + x)$$ Distribute the right side: $$0.3 + 0.25x = 0.45 + 0.15x$$ Subtract $0.15x$ from both sides: $$0.3 + 0.10x = 0.45$$ Subtract $0.3$ from both sides: $$0.10x = 0.15$$ Divide both sides by $0.10$: $$x = \frac{0.15}{0.10} = 1.5$$ 7. **Interpretation:** You must add 1.5 liters of the 25% saline solution to the 3 liters of 10% solution to get a 15% saline solution. **Final answer:** $\boxed{1.5}$ liters