James' car can travel 4 more miles on a gallon of gas than Robin's car.
The Breakdown
James' car can travel 80 miles on 4 gallons of gasoline, so the mileage per gallon for James' car is 80 miles / 4 gallons = 20 miles per gallon.
Robin's car can travel 80 miles on 5 gallons of gasoline, so the mileage per gallon for Robin's car is 80 miles / 5 gallons = 16 miles per gallon.
To determine the difference in mileage per gallon between the two cars, we subtract Robin's mileage per gallon from James' mileage per gallon:
20 miles per gallon - 16 miles per gallon = 4 miles per gallon.
Therefore, James' car can travel 4 more miles on a gallon of gas than Robin's car.
NB: Complete question
James estimated that his car can travel 80 miles on 4 gallons of gasoline, and Robin estimated that his car could travel the same distance on 5 gallons of gasoline. How many more miles can James' car travel on a gallon of gas than Robin's?