Answer:
the marginal revenue from selling the fifth unit is
$25
Step-by-step explanation:
Perfect competition is market structure in which the following information: 
 
All firms sell an identical product .
All firms are price takers , cannot influence the market price . 
Market share has no influence on prices. 
Buyers have complete informationabout the product being sold and the prices . 
Resources for such a labor are perfectly mobile. 
Firms can enter or exit the market without cost