Answer: $25
Explanation:
Total revenue, at price = $50
Total revenue = price × units sold
            = $50 × 0
            = 0
Total revenue, at price = $45
Total revenue = price × units sold
            = $45 × 1
            = $45
Total revenue, at price = $40
Total revenue = price × units sold
            = $40 × 2
            = $80
Total revenue, at price = $35
Total revenue = price × units sold
            = $35 × 3
            = $105
Marginal revenue of third unit = [tex]\frac{Change\ in\ total\ revenue}{change\ in\ units\ sold}[/tex]
                          = [tex]\frac{105 - 80}{3 - 2}[/tex]
                          = $25
