Answer: $48
Explanation: Marginal cost is the change in total cost when one more unit of output is produced.
Variable cost is that portion of total cost that changes with the level of output.
so, we can say that :-
variable cost = marginal cost of 1st unit + marginal cost of 2nd unit +
marginal cost of 3rd unit
= $20 + $16 + $12
= $48