Chapter 1 - Equations and Inequalities - 1.2 Applications and Modeling with Linear Equations - 1.2 Exercises - Page 103: 31

$2$ L

Set up a table: $\left[\begin{array}{ccc} \text{strength} & \text{L of solution} & \text{L of pure acid}\\ 100\% & x & x\\ 10\% & 7 & 0.7\\ & & \\ 30\% & (x+7) & 0.3(x+7) \end{array}\right]$ The equation to solve is: $ x+0.7=0.3(x+7)\qquad$ ... multiply with 10 $10x+7=3(x+7)$ $ 10x+7=3x+21\qquad$ ... add $(-3x-7)$ $7x=14$ $x=\displaystyle \frac{14}{7}=2$ L