Three masses labelled M1, M2 and M3 are stacked as shown in the figure for this question. M1 has a mass of 40 kg, M2 has a mass of 25 kg, and M3 has a mass of 11 kg. The coefficient of friction between the ground and Ml is μg1 =  0.15, and similarly between M1 and M2 is μ12 = 0.25 and between M2 and M3 is μ23 = 0.65. Determine the minimum value of F necessary to cause sliding and in the answer box below, circle the surface on which sliding occurs. 

M3 M2 20 N M1 30 N