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Let's write down what we know, the equation for finding the cost is as follows
where x=the number of belts.
We need to take the derivative of the above equation to find the rate of average change. Thus the derivative is as follows (let me know if you want to know how to take derivatives). C'(x)=14-.152x Then substitute in x=400 C'(400)=14-60.8 = -46.8 Thus they lose 46.8 dollars per belt. Please let me know if you need any clarification. I'm always happy to answer your questions.