The passage states that event A or event B will happen.
Let's use D or O to represent the developer and the owner, so we can say event D (the developer installs the system) or event O (the owner installs the system).
Now, this question works against the way most people use the word "or."
For example, if I say, "Would you like cake or pie for dessert?" am I saying that you can have cake or pie, but you cannot have both? Or am I saying that you can also have both if you wish?
What if I say, "Everyone on the committee is either an engineer or a Republican?" Does this allow for some people to be an engineer AND a Republican?

So, there are two types of "or." There is an inclusive "or" (which means that both things can be true) and an exclusive "or" (which means that both things cannot be true). On the GMAT, we must assume that "or" is always inclusive (unless stated otherwise)

So, on the GMAT, if a passage states "Carlee will go to Spain or France," we cannot rule out the possibility that she will visit both countries.

Now back to the question, here we are told that event D will occur or Event O will occur. We must assume that this is an inclusive "or" (unless told otherwise), which means it is possible that events D and O will both occue (the developer and the owner will work together to install the system).

The passage says that if event D occurs, how can we be certain that event O does not occur? The only way we can be certain is that if "or" in the passage is an exclusive "or" . Answer choice E says that "or" is exclusive"

mundasingh123 wrote:A developer has promised that either he or the owner will install a sewage system in a recently built house.
If the developer works to install the sewage system, it can be inferred that the owner does not help only if it is also true

• A)that the developer keeps his promises
  B) the developer often breaks his promises
  C)the owner does not want the sewage system
  D)the developer and the owner try to avoid working together to install the sewage system
  E) the developer's promise precludes him and the owner from both helping to install the sewage system