2. Cost of1DM ceramic tile: 8.64/0.0 1*0.6 yuan =5 18.4 yuan.

Similarly, the cost of 2DM, 3DM, 4DM and 5DM tiles is 540 yuan, 432 yuan, 324 yuan and 4 14.72 yuan respectively.

3. So the least cost is to use a square brick with a side length of 4dm.