At 2/3'd full, the tub is at 10.5. Meaning that all you would have to do is divide 10.5 by 2 to get what the tub would be like at 1/3'd full, which is 5.25. So it would take another 5.25 gallons to fill the tub up to 3/3, and the total gallons inside the tub would be 15.75.
