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I'm copying my question below as it is:

At a speed of S km per hour, a car will travel y km o each liter of petrol, where

y= 5+(1/5)S-(1/800)S^2

Calculate the speed at which the car should be driven for maximum economy.

The answer is 80 km/h; but how to get there from here? Thanks for the help in advance.

3 Answers 3