Answer: $30
Explanation: Marginal cost can be defined as the addition to the total cost when one more unit of output of service or good is produced. Marginal cost can be computed by dividing total cost with the difference of output at two levels. It can be shown as below :-
Marginal cost = change in total cost / change in output
where,
change in total cost = (120 * 2) - (120 * 1 ) = 120
change in total output = 9 - 5 = 4
therefore :-
[tex]Marginal\:cost\:=\:\frac{120}{4}[/tex]
= $30
