I don't know why ... I've done every single question in the text book up to this point, and am now stumped on one of the easier ones. I've made my equation, and solved it, but it is not the correct answer. Nor does the correct answer make any sense.(adsbygoogle = window.adsbygoogle || []).push({});

Either the back of the book is incorrect, or I have clearly misread the question. I'll show you what I did...

Stan needs to move some excess topsoil from his farm. He can hire a dump truck and a driver for $60/h. The driver will take 30 min to deliver a load of topsoil and return to the farm. One person will take 40 h to load the truck with soil. Labourers get $18/h (whether they are loading the truck with soil or waiting for the truck to return). How many labourers should Stan hire to minimize the cost per load?

Let h represent the number hours that one complete load will take. Let x represent the number of labourers that Stan is hiring to load the truck with soil. Let C represent the total cost of completing one load.

[tex]C = 60h + (18x)h[/tex]

Since the number of hours is determined by the number of labourers, plus half an hour of delivering the soil, the equation for h is as follows.

[tex]h = \frac{40}{x} + \frac{1}{2}[/tex]

Therefore, the solution to the equation is:

[tex]\begin{align*}

C &= 60(\frac{40}{x} + \frac{1}{2}) + (18x)(\frac{40}{x} + \frac{1}{2})\\

&= \frac{2400}{x} + 30 + 720 + 9x\\

\frac{dC}{dx} &= \frac{-2400}{x^{2}} + 9\\

x^{2} &= \frac{2400}{9}\\

x &\approx 16\\

\end{align*}[/tex]

Plugging x back in to the original equation will prove a minimum cost. However, the back of the book says the answer is 5. Why?

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# Homework Help: A Simple Calculus Question I Can't Get

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