Answer: Marginal cost under demand and supply theory. Answer is 80
Explanation: QD 100-4P, Marginal Cost =S4,QS =6P -20. So
the calculation goes thus = QS=6p-20
Inputing Marginal value of 4 equates 100-4(4)
100-16 = 84
QS=6(4)-4
24-20=4
profit maximisation =QD-QS
84-4=80