Question 64610
Let x=amount of water to be evaporated.

Now we know that the amount of pure salt remains the same before(32)(0.04) and after (32-x)(0.07) the evaporation process.  Thus, our equation to solve is:

(32-x)(0.07)=(32)(0.04) simplifying
-0.07x+2.24=1.28 subtract 2.24 from both sides and we have:
-0.07x=-0.96
x=13.71 oz 

ck
(32-13.71)(0.07)=(32)(0.04)
1.28=1.28

Hope this helps.  Happy holidays.------ptaylor