Since there are two cars in the Fleet,
there should be an odometer reading and a number of gallons for each car's
fillUp().
 fillUp()  Method
Assume that at the end of a week each car in the Fleet is filled with
gasoline and its new odometer reading is noted.
The fillUp() method for Fleet will get this data.
The first two parameters will be for the town car and the
last two will be for the sports utility vehicle.
Of course the Fleet.fillUp()  method is written in terms of the 
Car.fillUp()  method.
Fill up the blanks.