what about this exercise
20. A car traveled 462 miles per tankful of gasoline on the highway and 336 miles per tankful of gasoline in the city. If the car traveled 6 fewer miles per gallon in the city than on the highway, how many miles per gallon did the car travel in the city?
For the tankful the car travelled 126 miles fewer than on the highway.
Hence the tank had 21 gallons ( 126/6)
Hence miles/gallon in city = 336/21 = 16.