The amount of salt remians the same. Initially, there is 200 lbs of solution with 20 lbs of that being salt.
So, to get the 18%, \(\displaystyle 200-\frac{20}{0.18}\) has to be evaporated.
But, to set it up algebraically, let x=amount to be evaporated:
.18(200-x)=.10(200) and solve for x.