12 bags of plaster do 54m squared, how many bags are needed for 36m squared

This is a problem of proportions. Simply put, if x is the number of bags you're looking for, then 12/54 = x/36. Cross multiply to get 54x = 432, and thus x = 432/54 = 8. So the answer is 8 bags. Here's the theory behind it: If you divide the number of bags by the area covered, you are obtaining the number of bags required per unit area. If you then multiply that number of bags per unit area by the new area, you are left with the number of bags required to cover that area. In the proportion, I cross multiplied and divided by 54, but to be consistent with the theory you would take 12/54 and multiply it by 36 (this is just another way to solve the proportion). That was a long-winded answer to a simple question, but I think that if you can think about the theory behind the simpler math problems, it will be easier to work through more difficult ones later on.