The average cost (in Rs.) of producing x units of a commodity is \(AC = x + 5 + \displaystyle\frac{36}{x}\). The marginal cost when 4 units are produced is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Rs. 13
The total cost is the average cost times the number of units:
\( \displaystyle C(x) = x \times AC = x^2 + 5x + 36 \)
The marginal cost is the derivative of the total cost:
\( \displaystyle MC = \frac{dC}{dx} = 2x + 5 \)
At x = 4:
\(MC = 8 + 5 = 13\)
So the marginal cost is Rs. 13. Note that MC is found from C(x), not by differentiating AC.