Scientists in a test lab are testing the hardness of a surface before constructing a building. calculations indicate that the entire structure would sink by a certain amount for every additional floor that is added. if the maximum permissible limit for depression of the structure is 20 centimeters, how many floors can be safely added to the building?

Let's call this "certain amount" with a variable, x, (since it's not specified in the question, I will assume that the answer shall just be based on this variable). 

So every additional floor will make the building sink by x, and let's make the unit cm, so x cm. 

Now we know that the maximum depression is 20 cm, and it is also 

x*y, where y is the number of floors. 

So we have x*y=20, 

therefore the number of floors is:
20/x or                                                                                                                                      This is the answer