Question 373377
If you are ADDING WATER to dilute the mixture, then 
letting x = # of quarts water that had to be added, we get
0.65*18 + x = 0.80(x+18),
11.7 + x = 0.80x + 14.4,
0.20x = 2.7,
x = 13.5, # of quarts water that had to be added.