Hi there!
In order to solve this problem, we'll need to subtract the two quantities.
Here is the equation that we'll use to find out how much water is left: 8/10 - 4/10 = how much water is left. Next, we'll subtract 4 from 8 and since the denominators of both fractions are the same, we don't have to do any tricks! Subtracting the two values gives us: 4/10. We can simplify this to: 2/5 of a gallon of water is left.
Hope this helps!! :)
If there's anything else that I can help you with, please let me know!