This is an LSAT question. It is from the October 1994 exam, Logical Reasoning section 1, Question 25.
This is an example of formal logic that is possibly a bit beyond what is on the GMAT. I am going to explain this in formal logic as I would for the LSAT. You can diagram the stimulus this way:
If Actor ---> exuberant ----> extrovert.
Shy people some Actors.
Do you see what I have done? The arrow indicates the "therefore" or "then" statement. So we know that if someone is an actor he is exuberant and must also be an extrovert. So you can say that all actors are extroverts.
The other statement just indicates that some shy people are actors. Now, we can even but these statements together to say:
Shy people some Actors ---> exuberant ----> extrovert.
This means that some shy people are Actors and are therefore exuberant and extroverts. So we can actually say that some shy people are extroverts!
Use the diagram above on the answer choices to see which "must be true". Eliminate those that must be true on this "except" question.
A) Some shy people are extroverts. -- We just concluded this!
B) Some shy extroverts are not actors. -- Wait on this one it is not clearly true since we do not know about those who are "not actors" rather than analyze it too move on and come back if necessary. If the other four are eliminated this will be the correct answer.
C) Some exuberant people who are actors are shy. -- This is true because all actors are exuberant so if any shy people are actors they are exuberant people. And we know that there are shy people who are actors, so this must be true.
D) All people who are not extroverts are not actors. This is true. It is called the "contrapositive" Since we know that all actors are extroverts it must be true that those are fail to be extroverts cannot be actors.
E) Some extroverts are shy. This is true because you can always reverse a "some" statement. So If I say that "some judges are women" it is pretty clear that "some women are judges" as well. In this case is A is true then E is automatically true as well.
I checked and B is the OA. Does that help? Do you still have questions?
