Marginal analysis can be daunting because the problems have a few steps. But there are a few basic relationships you can use.
Revenue = Price * Quantity
Profit = Revenue – Cost
To estimate any marginal function, take its derivative. Here are a few examples from class.
Problem 1 Suppose the demand function is given by D(Q) = 0.05Q + 100 dollars per unit where Q is the number of units demanded by consumers.

 Find and interpret the marginal revenue at Q = 700 units.
 If the cost function is given by C(Q) = 9Q + 5650 dollars, find and interpret the marginal profit at Q = 700.
This tells us that the 701st unit increases the revenue by 30 dollars.
The 701st unit increases profit by 21 dollars.
Problem 2 Suppose the demand function is given by D(Q) = 0.01Q + 80 dollars per unit where Q is the number of units demanded by consumers.

 Find and interpret the marginal revenue at Q = 5000 units.
 If the cost function is given by C(Q) = 15Q = 50000 dollars, find and interpret the marginal profit at Q = 5000.
The 5001st unit decreases revenue by 20 dollars.
The 5001st units decreases profit by 35 dollars.