If E is measured in litres per 100 km, that is the reciprocal of efficiency. You do not want to maximize that number. You want to minimize it. The question does not make sense to me.
There is a relative maximum of E at v = 80 km/h, but that is a MINIMUM in fuel efficiency. The best efficiency is obtained as v-> 0, well below the speed limit.
The fuel efficiency, E, in litres per 100 km, for a car driven at speed v, in km/h is E(v)=1600v/(v^2+6400)
a) determine the legal speed that will maximize the fuel efficiency using speed limit of 50 km/h
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