I'll start with row 2. We have found the number of patients that gives us the greatest profit, just by comparing the price with the numbers in the marginal cost column. No arithmetic is needed. Now I'll do the arithmetic for you, to verify that this answer is correct. The marginal decision rule told you that without having to calculate out the whole table.
Competition Now let's introduce some competition.
They'll stay invisible, actually. Because there is profit to be made in this industry. Others can figure that out, and start their own companies just like Joan's. The result is that supply expands for the whole industry. If the industry demand curve doesn't change, the equilibrium price will fall.