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 Lā 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.