Chapter 1 - Section 1.7 - Modeling with Equations - 1.7 Exercises - Page 77: 58

2.4L

There are 6L of brine at a concentration of 120g/L That is the concentration of $\frac{720g}{6L}$ We want concentration of 200g/L The amount we need to boil off is x $\frac{720}{6-x}$=$\frac{200}{1}$ 720=200(6-x) 720=1200-200x 200x=480 x=2.4