6. A painter of mass 80kg stands on a horizontal non-uniform plank AB of length 4m and mass 20kg. The plank rests on supports at C and D, 1m from each end of the plank. The painter places pots of paint with mass 2.5kg 0.2m from each end of the plank. (a) When the painter stands at the centre of mass of the plank the reaction forces at C and D are in the ratio 4 1. Find the distance from A to the centre of mass." (b) he uses all the paint in the pot near A and discards the pot. He then stands on the plank at a point between D and B and finds that the plank is on the point of tilting about D. How far is he from B?