1/4 to 2/3
2/3-1/4=8/12-3/12=5/12
so the change is 5/12
the volume of the tank and dimentions are irrelivant for now
so
3000 cubic inches was added and that was a change of 5/12
5/12 of full=3000
times both sides by 12/5
full=7200
the capacity is 7200 cubic inches