Answer:
57.5 or 58
Explanation:
1150 = 20x
Divide both sides by 20 to get 57.5
To be proper, you should say that 58 gallons will be used.
It all depends on the teacher, but I know that mine used to tell me to round these types of things up. If you would like to be more exact, just put 57.5 gallons.