Surely you don't mean "input the kilometers driven and number of liters of gasoline used for each full tank". The number of liters per tank is not relevant. What is relevant is the total number of liters of gasoline used. Perhaps what you mean is "the number of full tanks". If you know how many liters there is in a tank (which is a constant) then multiplying that by the number of full tanks gives the number of liters of gasoline used. The "combined average gasoline consumption" is the total liters used divided by the distance driven.