* Must be true - CR

[pb]

Some good cooks are gourmet cooks who pride themselves on always using extravagantly rich ingredients in elaborate recipes. Some good cooks can be characterized as fast-food cooks. They may use rich ingredients as long as the recipes are easy to follow and take little time. Other good cooks are health food enthusiasts, who are concerned primarily with the nutritional value of food. But even though not all good cooks are big eaters, they all enjoy preparing and serving food.
If the information in the passage is true, which one of the following CANNOT be true?

(A) Most good cooks do not use extravagantly rich ingredients.

(B) Everyone who enjoys preparing and serving food is a good cook.

(C) More good cooks who use extravagantly rich ingredients are big eaters than are good cooks who do not use such ingredients.

(D) There are fewer good cooks who enjoy serving and preparing food than there are good cooks who are big eaters.

(E) Gourmet cooks, fast-food cooks, and cooks who are health food enthusiasts are all big eaters.

Why is B not correct? OA is D

[jnelson0612]

Let's first look at why D is correct. The problem says: But even though not all good cooks are big eaters, they all enjoy preparing and serving food.

Let's plug in real numbers to evaluate this. We'll assume that we have 100 good cooks. This statement implies that all 100 enjoy preparing and serving food. However, not all good cooks are big eaters. Thus, good cooks/big eaters are less than 100, or 99 or below. That goes exactly opposite to what D says; thus D cannot be true.

Your answer, B, is a reversal of what is said in the sentence. What I know is that all good cooks enjoy preparing and serving food. Thus, if I am a good cook I enjoy preparing and serving. Can I say the opposite--if I enjoy preparing and serving that I am a good cook?

This concept (If X then Y, thus If Y then X) is not correct in logic. I'll give you an easy example:
If I am a woman, then I am a female. (correct)
Can I conclude that if I am a female that I am a woman? No, I may be a girl.

Please just remember that "If X, then Y" DOES NOT logically reverse to "If Y, then X" and that you can plug and test numbers in examples such as this one.

[thulsy]

Dear instructors,
I understand D is correct, but how could I eliminate E?

the stimulus says "even though not all good cooks are big eaters".
Since "good cooks" is a subset of "cooks", I reckon that the statement of E (cooks - divided in three complete subsets - are ALL big eaters) cannot be true.

Thanks.

[boaz.khan]

My 2 cents on (E):

Choice E does NOT specify how many of the three types of cooks are big eaters. It could be that the statement is only referring to a handful of these three types, which very well could be true.

hope this helps.

[jnelson0612]

My 2 cents on (E):

Choice E does NOT specify how many of the three types of cooks are big eaters. It could be that the statement is only referring to a handful of these three types, which very well could be true.

hope this helps.

Correct! I would also be wary of the word "all", which is quite extreme, in this type of a question.

[thulsy]

thanks boaz.khan and Jamie for your reply.
sorry I'm a little lost here. I'm trying to understand why E is incorrect, i.e. why E could be true.

boaz.khan says

Choice E does NOT specify how many of the three types of cooks are big eaters. It could be that the statement is only referring to a handful of these three types, which very well could be true.

does that mean:

"Students all love studying. "
Although of course not all the students IN THE WOLRD love studying, this statement could be true because "students" does not specify the range/scope of students under consideration. "students" could be a group of students who love studying.
Am I correct?

[RonPurewal]

this looks suspiciously like an LSAT problem. this kind of formal logic ("all x's are y's, some a's are b's, if c then not d, etc") is not tested on the gmat.

what is the actual source of this problem?
if this problem is actually from the OFFICIAL GMAT PREP SOFTWARE, please post a screenshot from the software as proof.
if no such screenshot is posted within about a week, we will delete this thread. thanks.

[Tadashi]

Yes you are right,Ron.
the question is from LSAT in 90's.
I am quite sure that you can delete that thread.
really not a good problem for a gmat test taker to learn from....haha.
Tadashi

[RonPurewal]

Thanks.

Don't use LSAT logical reasoning problems, people. They'll largely condition you against using exactly the kind of (non-rigorous, common-sense) reasoning required by most GMAT CR problems.