Aedifico Ltd is a small construction firm. The total cost (TC, in £) and marginal cost (MC) of Aedifico's concrete depends on tons produced (Q) as follows:
TC = Q³ - 9Q² + 40Q + 50
MC = 3Q² – 18Q + 40
Marginal cost and average cost (TC/Q) have been plotted here. To see their relationship with the total cost curve, see http://www.geogebratube.org/student/m13345

When is AC greater than MC? When is AC less than MC? Where is the smallest possible AC?