Suppose that a pure monopolist can sell 20 units of output at $10 per unit and 21 units at $9.75 per unit. the marginal revenue of the 21st unit of output is:

Marginal revenue is defined as the amount that you gained after selling all your units at a certain price. Revenue is different from profit, because profit has to incorporate the expenses incurred in order to produce the product. For total revenue, that would just represent the total sales of a firm or company. However, marginal revenue is the additional cost a consumer has to pay when he acquires an additional unit of the product. Thus, marginal revenue is the change of sales per unit product.

Marginal Revenue = ΔRevenue/ΔNumber of units
Marginal Revenue = [21($9.75) - 20($10)]/(21-20)
Marginal Revenue = $4.75 per unit