Question 99099
YOU ARE CORRECT. THE WORDING SHOULD BE "ADDED" NOT "EVAPORATED".  THE MORE WATER EVAPORATED, THE STRONGER THE SOLUTION.  IF WE LOOK AT THE LIMITING CASE: WHEN WE EVAPORATE ALL THE WATER, WE HAVE PURE SALT LEFT.

Let x=amount of water that needs to be added

Now we know that the pure salt in the 50 liter solution (0.65*50) plus the pure salt in the water added (0) has to equal the pure salt in the final mixture(0.40)(50+x).  So our equation to solve is:

0.65*50=0.40(50+x) get rid of the parens

32.5=20+0.40x  subtract 20 from both sides

32.5-20=20-20+0.40x  collect like terms

12.5=0.40x  divide both sides by 0.40

x=31.25 liters -------------amount of pure water needed

CK
0.65*50=0.40*81.25

32.5=32.5


Hope this helps----ptaylor