Homework Help: Marginal profits

1. Feb 14, 2010

bacon

This question is not homework(I'm not even in school). Also, I know how to get the answers but have a question regarding the reasonableness of an answer. This is taken from a calculus book in the chapter introducing the derivative.
Problem: It costs $$C(x)=50+\frac{1}{4}x-\frac{1}{30}x^{2}$$ dollars to make x dozen donuts in one day $$({0}\leq{x}\leq{40})$$ and the donuts sell for 75 cents a dozen. a) What are the Revenue R(x) (gross income) and profit P(x) on x dozen donuts? b) What are the average and marginal profits on 30 dozen donuts?
answers a) $$R(x)=.75x dollars ; P(x)=\frac{1}{2}x + \frac{1}{30}x^{2}-50dollars$$ b)$$Averageprofit=-\frac{1}{6}dollar/dozen; Marginal profit=\frac{5}{2}dollars/dozen.$$