Final answer:
The marginal revenue for the 6th unit is the additional revenue from selling that unit, calculated as $8. Additional information is needed to determine the marginal revenue for the 8th unit.
Step-by-step explanation:
The question relates to the concept of marginal revenue in economics, specifically within the context of a monopolistic market. Marginal revenue is the additional revenue that a firm gains from selling one more unit of a good or service. According to the information given, a monopolist sells the 5th unit for $14 and the 6th unit for $13. To calculate the marginal revenue of the 8th unit, one needs additional information about the price at which the 7th and 8th units can be sold. However, based on the information provided, we can calculate the marginal revenue for the 6th unit, which is the difference in total revenue obtained by selling 6 units instead of 5 units, which is $13 (price per unit) x 6 units - $14 (price per unit) x 5 units = $78 - $70 = $8.