puzzled :(
The cost of operating a truck on a highway is made up of running costs plus river's wage - the running costsss are (0.25 + s/250) dollars/kilometre, where S is the speed of the truck in km/h, and the driver's wage is $45 per hour. (a) write an expression for the driver's wage for a 750km trip in terms of S (b) write an equation for the total cost of a 750km trip (c) at what speed should the driver travel to make the 750km trip as economical as possible
Note that as, $S$ is the speed of the truck, then using the formula, Distance= Speed $\times$ Time, we get, $$\text{ Total Cost } = \text{ Running Cost } + \text{ Driver wages } $$ $$= (\frac{1}{4}+\frac{s}{250})\times 750 + 45\times \frac{750}{S}$$
Hope you can take it from here.