Choice B is the correct answer. Let $x$ = liters of 40% solution.

Equation: $0.10(30) + 0.40x = 0.25(30 + x)$$3 + 0.40x = 7.5 + 0.25x$$0.15x = 4.5$$x = 30$

Solution: 30 liters

Verification: $\frac{0.10(30) + 0.40(30)}{30 + 30} = \frac{3 + 12}{60} = \frac{15}{60} = 0.25$ ✓

💡 Strategic Tip: Amount of solute = concentration × volume.