SOLUTION: I mix one 12 fluid ounce can of concentrated juice with 4 1/3 cans of cold water. If I have a 1 gallon pitcher, how many more fluid ounces of concentrated juice is needed to fill

We must calculate the number of fluid ounces of drinkable juice we now 
have in the pitcher.

Let's first calculate the number of fluid ounces of water we now have
in the pitcher:
                             
           

That's 52 fluid ounces of water plus the 12 fluid ounces of 
concentrated juice.  So that's 52+12 = 64 fluid ounces of drinkable
juice in the pitcher. 

1 gallon = 128 fluid ounces.  Therefore the pitcher is exactly half full,
since 128 = 2×64.

Therefore to fill the pitcher we need to do again what we just did.

We need 12 fluid ounces more of concentrated juice to mix with another
 cans of cold water.

Edwin