Choice B is the correct answer. Let $x$ = liters of 25% solution to add.

Equation: (amount of salt) before mixing = (amount of salt) after mixing $0.10(20) + 0.25x = 0.18(20 + x)$

Step 1: Expand: $2 + 0.25x = 3.6 + 0.18x$

Step 2: Solve: $0.25x - 0.18x = 3.6 - 2$$0.07x = 1.6$$x = \frac{1.6}{0.07} = \frac{160}{7} \approx 22.86$

$0.10(20) + 0.25(20) = 2 + 5 = 7$ total salt Total solution: $20 + 20 = 40$ liters Concentration: $\frac{7}{40} = 0.175 = 17.5\%$ ✗

$2 + 0.25x = 0.18(20 + x)$$2 + 0.25x = 3.6 + 0.18x$$0.07x = 1.6$$x = 22.857...$

💡 Strategic Tip: Mixture problems: concentration × volume = amount of substance.

**Other choices based on calculation.