This problem feels pretty straight forward, but I always like to double check. Thus far I have:

For all parts, consider the cost function:

A. Give the extra cost of producing one more unit when the production is already at a level of x=50

I gave $5.51 because C(51)-C(50)=5.51

B. Give the marginal cost function.

I gave

C. Give the average marginal cost function.

This one I'm having problems with (more so if I did the first two wrong), but I think it has something to do with a summation formula.

I was thinking something along the lines of

Or something... I really don't know. I think I'm pretty good with the whole margins thing and averages were never hard, but I'm looking at this one with a bit of a blank. After reading through the basic summations I'm getting that I need some n and n+1, but I'm not sure how to lay it out.