Order of operations using "gemdas" solve (step by step) (algebra 2)

all work is shown and pictured

You take 1/3, which is the amount you are looking for to fill the rest of the container, then you multiply that times 1/2.  this equals 1/6 of a gallon. then you add that to 1/2 a gallon to find out the total of all the water it takes to fill the bin.  1/2+1/6=3/6+1/6=4/6 or 2/3 gallon simplified.