The container used to be 1/4 full, nd when you put in 9 gallons, it is 5/8 full.
First, we should have the fractions have the same denominator
1/4 = 2/8
5/8
We subtract
5/8 - 2/8 = 3/8
So if we add in 9 gallons, 1/8 of the container would be 3 gallons.
2/8 = 6 gallons
6 + 9 = 15 gallons
8/8 - 5/8 = 3/8
3/8 = 9 gallons
15 + 9 = 14 gallons