Critical Reasoning

28. (1) All A are B.
(2) Some B are C.
Which of the following is true?
(A) All A are C.
(B) No A are C.
(C) Some A are C.
(D) No C are A.
(E) None of the above.

I picked C. correct ans[color=white] E[/color]

please advise, my logic was that if a is part of b and some b is part of c, it is possible for some a to be part of C. why is that incorrect?

Hello,

In such questions the most important point is to imagine a situation that encapsulates every possibility. You will see in the attached image the Venn Diagram that satisfies both the conditions and satisfies every possibility of interpretation.

shahpuja wrote:28. (1) All A are B.
(2) Some B are C.
Which of the following is true?
(A) All A are C.
(B) No A are C.
(C) Some A are C.
(D) No C are A.
(E) None of the above.

I picked C. correct ans E

please advise, my logic was that if a is part of b and some b is part of c, it is possible for some a to be part of C. why is that incorrect?

Venn diagrams are one way we can keep track of this type of information, although the diagram provided by ikant is actually NOT an accurate representation.

Let's examine the two rules we have:

(1) all A are B.

From this, we know that if something is A, it's also B.

(2) Some B are C.

From this, we know that there's at least one B that's also C.

If we put the two statements together, we don't know anything for sure. Could some A also be C? Sure, but it's not definite.

Let's use a real-life example to make that clear:

All Dogs are Mammals.

Some Mammals are Cats.

Does this mean that some dogs are cats? Of course not!

Going back to your point, it may just be that you misinterpreted the question.

"Which of the following IS true?" means that we're looking for something that MUST be true. In your own words, all that we can conclude is

it is possible for some a to be part of C

which doesn't match what the question is asking.

Hi,

You are right..I just realised the mistake I did in drawing the diagram. There is actually no single Venn diagram possible to be drwan for this preposition.

...

I dont remember this format of the question in any GMAT material. I have seen the question in IIM- CAT India. The question certainly is triggering you to think critically. But the real trick in GMAT is how you decipher the question and reach this stage (as stated in question). Has anyone seen this type of question in real GMAT?

Thanks.
~R

No i have also not seen such questions i agree with rajmirra.

This is much more of an LSAT style than GMAT style question. It's very rare for the GMAT to test you on formal logic.

Thanks stuart for explanation but please provide some guidance where i can find such questions just for general practice. :

michelsmithm wrote:Thanks stuart for explanation but please provide some guidance where i can find such questions just for general practice. :

If you really want to practice this type of question (which is fairly rare on the GMAT), then I'd suggest LSAT materials which focus on the logical reasoning section of that test - especially inference questions.

If you want a better understanding of formal logic (and you only need to know the VERY basics, even for the LSAT), I'd look for a primer on if-then statements. The formal logic tested on the GMAT is minimal and even on the LSAT it's only what one would learn in the first week of a university level course.

shahpuja wrote:28. (1) All A are B.
(2) Some B are C.
Which of the following is true?
(A) All A are C.
(B) No A are C.
(C) Some A are C.
(D) No C are A.
(E) None of the above.

I picked C. correct ans E

please advise, my logic was that if a is part of b and some b is part of c, it is possible for some a to be part of C. why is that incorrect?

E is the correct answer.

You should draw a venn diagram to solve this type of question.

Here it is

I would choose A. But i am not sure of my answer.