a) A customary measure of fuel economy is L/(100 km). To find those values, you can divide [26.8, 23, 11.25, 33.25, 16.2] by [4, 2.5, 1.5, 3.5, 2] to get
.. [6.7, 9.2, 7.5, 9.5, 8.1]
(Many graphing calculators are able to handle lists, so this can be done "all at once.")
By these numbers, the Toyota Echo is the most economical at 6.7 L/(100 km).
b) The least economical is the Saturn Ion at 9.5 L/(100 km). That car uses
.. (9.5 -6.7)L/(100 km) = 2.8 L/(100 km)
more fuel. So at a distance of 500 km, the extra fuel required by the Ion will be
.. (2.8 L)/(100 km) * (500 km) = 14 L
The fuel economy is assumed to be constant at the given values over the 500 km range.