Final answer:
The ratio of the number of gallons of oil to the number of gallons of solvent is 2:3.
Step-by-step explanation:
The ratio of the number of gallons of oil to the number of gallons of solvent can be determined by subtracting the number of gallons of oil from the total volume of the mixture and then dividing by the number of gallons of solvent. In this case, the total volume of the mixture is 45 gallons and the number of gallons of oil is 18. So, the number of gallons of solvent is 45 - 18 = 27. Therefore, the ratio of the number of gallons of oil to the number of gallons of solvent is 18:27, which can be simplified to 2:3.