Since it is given that marginal cost is the instantaneous rate of change of total cost, we have
→dC/dx
→dC/dx = 3 × 0.05 × x² - 2 × 0.02 × x + 30 = 0.15x² - 0.04x + 30
Given in the question, the marginal cost of 3 units is to be calculated, so x = 3.
→Marginal Cost = 0.15(3)² - 0.04(3) + 30 = 1.35 - 0.12 + 30 = 31.23