Final answer:
The marginal revenue earned from selling the 9th unit is $13, which is the difference between the total revenue from selling 9 units at $21 each and 8 units at $22 each.
Step-by-step explanation:
To calculate the marginal revenue earned from selling the 9th unit, we need to analyze the change in total revenue from selling one additional unit. When 8 units are sold at $22 each, the total revenue is $22 x 8 = $176. When 9 units are sold at $21 each, the total revenue is $21 x 9 = $189. The marginal revenue of the 9th unit is found by subtracting the total revenue at 8 units from the total revenue at 9 units.
MR = Total Revenue from 9 units - Total Revenue from 8 units
MR = $189 - $176
MR = $13
Therefore, the marginal revenue earned from selling the 9th unit is $13.