1. **State the problem:** A chemist wants to mix a 10% acid solution and a 25% acid solution to get 60 liters of a 15% acid solution. We need to find how many liters of each solution to mix. 2. **Define variables:** Let $x$ be the liters of 10% acid solution and $y$ be the liters of 25% acid solution. 3. **Write the system of equations:** - Total volume: $$x + y = 60$$ - Acid concentration: $$0.10x + 0.25y = 0.15 \times 60$$ 4. **Simplify the acid concentration equation:** $$0.10x + 0.25y = 9$$ 5. **Solve the system:** From the first equation, $$y = 60 - x$$. Substitute into the second: $$0.10x + 0.25(60 - x) = 9$$ $$0.10x + 15 - 0.25x = 9$$ $$-0.15x + 15 = 9$$ $$-0.15x = -6$$ $$x = \frac{-6}{-0.15} = 40$$ 6. **Find $y$:** $$y = 60 - 40 = 20$$ 7. **Interpretation:** Mix 40 liters of 10% acid solution and 20 liters of 25% acid solution to get 60 liters of 15% acid solution. **Answer:** 40 liters of 10% solution and 20 liters of 25% solution. This corresponds to option c (40 liters of 10% solution).