If the bucket is half full and still has half to go, you double the current contents of the bucket. 3 4/6 times 2 is equal to 7 2/6 gallons which simplifies to 7 1/3 gallons.
I don't fully understand your question though, if the question is asking how much MORE water is needed to fill the bucket, it's 3 2/3 gallons. If it's asking how much water in TOTAL it would take to fill the bucket completely, it's 7 1/3 gallons.
