The factor is 2. Just think, what is 20 divided by 10? The answer is 2.
Answer:
The car's average 18 number of miles per gallon
Step-by-step explanation:
The car has different efficiencies depending if its used on highway or city streets.
The efficiency on a highway is:

The efficiency for city driving is:

We now that the car uses twice as many gallons for city driving as for highway driving. This means that, for every gallon consumed, 2/3 are for city driving and 1/3 are for highway driving.
Then, we can calculate how many miles makes the car on average as:

12 ft * 10 ft = 120 sq ft
½ * 6 ft * 7 ft = 21 sq ft
120 sq ft - 21 sq ft = 99 sq ft (it is area of the composite shape)