Answer
Marginal cost is the change in total cost of production in a business when producing one more unit of a product. It is the instantaneous rate of change of the cost.
Work Step by Step
Marginal cost is calculated by dividing the change in total cost by the change in quantity.
$$Marginal\;cost=\frac{Change\;in\;total\;cost}{Change\;in\;quantity}$$
Example: If at first, a factory produces a batch of $50$ units at a cost of $\$1000$, then it later produces an additional batch of $50$ units at a cost of $\$900$. $$Marginal\;cost=\frac{900}{50} =\$18\;per\;unit$$
Note: The difference between the cost of producing the first batch and the additional batch is the fixed cost which remains constant.
