The cost of making $x$ units is $C(x) = 100 + 40\sqrt{x}$ rupees. What is the marginal cost when 25 units are made?

Explanation

1. 01

   $C'(x) = 40 \times \frac{1}{2\sqrt{x}} = \frac{20}{\sqrt{x}}$.

2. 02

   At $x = 25$: $\frac{20}{5} = 4$ rupees.