There are several information's that are already given in the question. Based on those information's the answer to the question can be easily deduced.
Weight of 4 gallons of gasoline = 25 pounds
Then
Weight of 1 gallon of gasoline = 25/4 pounds
= 6.25 pounds
From the above deduction, we can easily conclude that the unit rate in pounds per gallon of gasoline is 6.25 pounds/gallon. I hope the answer helps you.