Choice B is the correct answer. Let $x$ = liters of pure alcohol (100%) to add.

Equation: Amount of alcohol before = amount after $0.15(60) + 1.00(x) = 0.25(60 + x)$$9 + x = 15 + 0.25x$$0.75x = 6$$x = 8$

Solution: 8 liters of pure alcohol

Verification: $\frac{9 + 8}{60 + 8} = \frac{17}{68} = 0.25 = 25\%$ ✓

💡 Strategic Tip: Pure substance = 100% concentration.

**Other choices fail verification.