A friend of mine is having a bit of difficulty with their calculus homework and came to me for help, but I'm stumped at this point. The question is asking for the marginal profit of the 31st unit of a good, which I understand to be $P'(31)=R'(31)-C'(31)$, where $P'(x), R'(x),$ and $C'(x)$ are marginal profit, marginal revenue, and marginal cost, respectively.
The question gives that the price of a unit of the good as 215 dollars and gives the marginal cost function $C'(x)$. It also gives that there is a baseline manufacturing cost for any number of units of good, but my understanding is that this value is irrelevant to marginal profit. So the marginal profit of the 31st good would thus be $P'(31)=215-C'(31)$, yes? We've run the question several times but the online homework platform keeps marking it as wrong.
I suppose I should add that I've never encountered marginal profit before today, so I may just be misunderstanding something.