From the first equation, $ y = 20 - x $. Substitute into the second: $ 0.15x + 0.05(20 - x) = 2 $. Simplify: $ 0.15x + 1 - 0.05x = 2 $ → $ 0.10x = 1 $ → $ x = 10 $. Then $ y = 10 $. Thus, $ \boxed{10} $ liters of each solution are needed.

["Solving Simple Linear Equations: A Step-by-Step Guide Using Real-World Context", "Understanding how to solve linear equations is a foundational math skill with practical applications in science, engineering, and everyday decision-making. Let’s walk through a classic problem involving substitution and simplification to find a precise solution—perfect for students, educators, and problem solvers alike.", "### Starting Point: The First Equation\nWe begin with the equation:\n$$ y = 20 - x $$\nThis expresses $ y $ in terms of $ x $, illustrating an inverse relationship common in real-world scenarios like mixing solutions or balancing budgets.", "### Substitution: Plugging into the Second Equation\nThe next step involves substituting $ y = 20 - x $ into a second equation:\n$$ 0.15x + 0.05(20 - x) = 2 $$\nThis substitution simplifies the problem by replacing $ y $ with an equivalent expression, allowing us to solve for $ x $ directly.", "### Simplifying the Equation\nDistribute and combine like terms:\n$$\n0.15x + 0.05 \cdot 20 - 0.05x = 2\n$$\n$$\n0.15x + 1 - 0.05x = 2\n$$\n$$\n(0.15x - 0.05x) + 1 = 2\n$$\n$$\n0.10x + 1 = 2\n$$", "### Solving for $ x $\nSubtract 1 from both sides:\n$$ 0.10x = 1 $$\nDivide both sides by 0.10:\n$$ x = 10 $$", "### Back-Substitution to Find $ y $\nNow that $ x = 10 $, substitute back into the first equation:\n$$ y = 20 - x = 20 - 10 = 10 $$", "### Final Answer\nThus, the solution to the system is:\n$$ \boxed{10} \ ext{ liters of each solution are needed.} $$", "### Why This Works (and Why It Matters)\nThis method combines substitution with algebraic simplification—essential techniques for solving systems of equations efficiently. These skills are vital when modeling chemical reactions, determining equilibrium concentrations, or optimizing resource allocation. Mastering linear equations lays the groundwork for more advanced math, including calculus and linear algebra.", "### Practice Tip\nTry substituting into similar equations, such as $ 2x + y = 15 $ and $ y = 5 - 2x $, to reinforce your solving skills. With practice, substitution becomes second nature—turning complex problems into simple, solvable steps.", "---", "Investing time in understanding equations like $ y = 20 - x $ and mastering substitution transforms abstract math into powerful real-world tools. Whether balancing equations in chemistry or strategizing budgets, clarity in solving linear systems stands as a cornerstone of logical thinking. Start today—your next solution begins with $ y = 20 - x $, one step at a time."]