Choice B is the correct answer. Let $x$ = liters of 20% solution and $y$ = liters of 50% solution.

Step 1: Set up the system: $\begin{cases} x + y = 30 \\ 0.20x + 0.50y = 0.35(30) \end{cases}$

Step 2: Simplify second equation: $0.20x + 0.50y = 10.5$

Multiply by 10: $2x + 5y = 105$

Step 3: From first equation: $y = 30 - x$

Substitute: $2x + 5(30 - x) = 105$$2x + 150 - 5x = 105$$-3x = -45$$x = 15$

Solution: 15 liters of 20% solution (and 15 liters of 50% solution)

Verification: $15 + 15 = 30$ ✓ and $0.20(15) + 0.50(15) = 3 + 7.5 = 10.5 = 0.35(30)$ ✓

💡 Strategic Tip: In mixture problems, (concentration × volume) gives the amount of pure substance.

Choice A is incorrect because $0.20(10) + 0.50(20) = 2 + 10 = 12 \ eq 10.5$.

Choice C is incorrect because $0.20(18) + 0.50(12) = 3.6 + 6 = 9.6 \ eq 10.5$.

Choice D is incorrect because $0.20(12) + 0.50(18) = 2.4 + 9 = 11.4 \ eq 10.5$.