Average and marginal cost on total cost curve

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 The total cost curve has been plotted here, and the average cost (gradient of ray) and marginal cost (gradient of tangent) indicated. To see the marginal and average costs per ton graphed on their own axis, see http://www.geogebratube.org/student/m13348
When is AC greater than MC? When is AC less than MC? Where is the smallest possible AC?