Answer
8 liters
Work Step by Step
Let's say that $s$ is how much $70$% acid is used, and $f$ is how much $45$% acid is used. She needs 20 liters of $60$% acid.
$0.7s + 0.45f = 0.6 * 20$
The student will need to combine them to make a 20 liter solution, so the amount of each solution used combined will be 20 liters.
$s + f = 20$
We can now make a system. Solve this system using any method. I will use graphing. Since I am graphing, I will use $s$ as the variable for $x$ and $f$ as the variable for $y$. The point I got is $(12,8)$. This means that e 12 liters of $70$% acid and 8 liters of $45$% acid will be used.
