1. **State the problem:** We have 9000 liters of a 30% acid solution. We want to add water to reduce the acid concentration to 20%. 2. **Find the amount of acid in the initial solution:** The solution is 30% acid, so acid volume is $9000 \times 0.30 = 2700$ liters. 3. **Let $x$ be the amount of water added in liters.** After adding water, total volume becomes $9000 + x$ liters. 4. **Set up the equation for the desired concentration:** The amount of acid remains the same (2700 liters), but the total volume is now $9000 + x$. We want concentration to be 20%, so: $$\frac{2700}{9000 + x} = 0.20$$ 5. **Solve for $x$: ** $$2700 = 0.20(9000 + x)$$ $$2700 = 1800 + 0.20x$$ $$2700 - 1800 = 0.20x$$ $$900 = 0.20x$$ $$x = \frac{900}{0.20} = 4500$$ liters. 6. **Interpretation:** We must add 4500 liters of water to get a 20% acid solution.