Answer:
Dependent variable = Money left
Independent variable = Gallons pumped
Explanation:
When finding the dependent vs independent variable, you usually want to see what is being plugged in to the function. This is oftentimes the independent variable, as it is not influenced by anything involved with the function, and acts on it's own.
The dependent variable is usually what is being set equal to the expression being solved, in this case that is the money that you have left, which goes down as you pump more gas. This is entirely dependent on how much gas is used.
Hope this helps, if you need any more explanation or help please let me know.