To find the car with the greatest efficiency, we'll compare the fuel efficiency (in km/l) and consider the weight of the cars.
- 900 kg car that travels 20 km/l: Efficiency = 20 km/l
- 1,200 kg car that travels 20 km/l: Efficiency = 20 km/l
- 1,200 kg car that travels 15 km/l: Efficiency = 15 km/l
- 900 kg car that travels 15 km/l: Efficiency = 15 km/l
Both the 900 kg car and the 1,200 kg car that travel 20 km/l have the same fuel efficiency, but the 900 kg car is lighter.
While the weight does not directly affect the fuel efficiency numbers given, typically, a lighter car has potential for better overall efficiency in practical terms. However, since both lighter cars achieve the same fuel economy figures, the conclusion is that the cars with the greatest efficiency in terms of fuel consumption are the two that travel 20 km/l.
The answer is: Either the 900 kg car that travels 20 km/l or the 1,200 kg car that travels 20 km/l (both have the same fuel efficiency of 20 km/l).