Assume a student told an instructor that he needed an "A" in the class. What shape would his demand curve have? why isn't this realistic?
I guess you would have your y axis be "price" in terms of amount of time studying in this case. In that case, the demand curve would be vertical -- at any price, the student only wants one outcome -- an A.
For any other outcome, his quantity demanded would be zero.
I suppose the most likely reason why this is not realistic is that no one really wants an A so badly as to spend literally all of their time studying for that one particular class. There would surely come a point when the student decided that the marginal return on his studying was no longer worth the marginal cost.
It is not an unrealistic question. The theory of mathematical economics is as well applicable to this type question also. When the availability of knowledge is hardly available the price to pay this type of demand may be high. Even if the tutor is single he may price his tutoring price at a higher level as the demand of getting an A goes high, making a strict condition on the student that the tutor charges the higher price and he wil do the best in pulling up the student by giving the best of his efforts - But his duty is only to pull the thirsty horse with better better efforts till the water and to drink or not is the dicision of the horse.