In practice, a firm would probably proceed in the reverse order from the way we did. It would understand that total cost is made up of fixed cost and variable cost. It would figure out what its fixed cost is and what its variable costs are at different rates of output. Then it would add the fixed and variable cost to get the total cost. Now let's introduce marginal cost.
Marginal cost is the difference in total cost between one rate of output and another.
Usually, unless stated otherwise, the marginal cost is the change in cost that results from changing the output by one unit. Here's the top part of the cost table, with a column added for marginal cost. I've left spaces between the lines for the marginal costs. This emphasizes that marginal cost is the difference in total cost between one output rate and another. Click on an answer box.