a) Divide the liters by the number of hundred km to find the fuel usage for each vehicle. The results are shown in my calculator's L₃ list. The Toyota Echo has the best fuel economy (least liters of fuel use for 100 km).
b) The worst fuel economy is 9.5 L/(100 km); the best is 6.7 L/(100 km). The difference is 2.8 L/(100 km) or 14 L/(500 km).
The assumption is that these fuel economy numbers remain accurate over the distance of 500 km.