This question involves carefully considering which parts have direct proportionality and which parts have inverse proportionality. Increasing the number of people should reduce the time involved, and reducing the number of walls to be painted also reduces the time. So the fraction involving people is 4/9 and the fraction involving the walls is 6/8 or 3/4. The calculation is 4/9*3/4*30=10 minutes. (Assuming they are well organised!)