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Since we have been working on some logical reasoning

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05 Jun 2005, 18:32
Take this statement the understanding will be lot easier:

If it rains then it is cloudy.(X ---> Y)

Clearly "it is cloudy" is the necessary but not sufficient for rain.

If it is cloudy then it does not mean it will rain.(Y does not -----> X)

But if it's not cloudy then it won't rain.(~Y -----> ~X)
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12 Jul 2005, 21:56
woow..wooww

what a POST...

Thanks a BILLION
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More on Some and All [#permalink]

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18 Aug 2005, 06:50
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Paul has made a very good example to show the relationship between some and all.
Paul wrote:
Which of the following conclusions can be deduced from the two statements below?

Some Alphas are not Gammas
All Betas are Gammas

A) Some Alphas are not Betas
B) No Gammas are Alphas
C) All Gammas are Betas
D) All Alphas are Gammas
E) Some Alphas are Gammas

What we know:
Some A are not G. And all B are G.

What can we infer from here?
Very limited. If it is an A then it may or may not be G. If it is B then it is definitely G. If it is a G then it may or may not be A, and it may or may not be B. If it is not G then it must not be a B.

What can we NOT infer from here?

Some A are G. -- That's very plausible, but very WRONG! We know that some A are not G, but it is possible that all A are not G, we just don't know. For example: some snakes do not have feet. From this statement, can we conclude that some snakes do have feet? NO.

By the same token, we can't say some G are A. We don't know if some animals with feet are snakes.

Some G are not A. -- Again this is wrong. Although some A are not G, but perhaps G only include the As that is G. Say some integers are odd numbers. Obviously it would be wrong to say some odd numbers are not integers.

All G are B. -- This is pretty obvious. All odd numbers are integers but not all integers are odd.

Now lets look at the statement one by one.

A) Some Alphas are not Betas
We know that some A are not G. And if it is not G then it must not be B. Therefore some A are not B.
Correct

B) No Gammas are Alphas
We konw some A are not G. It is possible that some A are G, and it is also possible that no A is G. We just don't know. If no A is G then no G is A, but if some A are G then some G must be A. Some integers are not odd, but it is incorrect to say that no odd numbers are integers.
In other words this is NOT always true.

C) All Gammas are Betas
We know that all B are G, but we don't know if all G are B. All odd numbers are integers but not all integers are odd.

D) All Alphas are Gammas
Obviously wrong because we know some A are not G.

E) Some Alphas are Gammas
We don't know about it. It may or may not be true. Perhaps some A are G. Perhaps no A is G. All we know is some A are not G.
Say A=integers, G=odd numbers. Some integers are not odd numbers, but some integers are odd numbers.
Now let's say A=even numbers, G=odd numbers. Some even numbers are not odd numbers. But it will be wrong to say some even numbers are odd numbers.
The key here is when we make a statement about "some", we are not saying anything about the rest. The rest may or may not be different from the "some" that we have made a statement about.
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More discussions on if ... then ... [#permalink]

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19 Aug 2005, 08:29
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More discussions on if ... then ...

Question:
If i say X happens only because of Y is that the same as saying X if and only if Y

No. X happens only because Y happened, in other words, if Y does not happen then X will not happen. That means X->Y. If X happened, we know for sure that Y has happened. But we don't know if Y happens whether X will happen. So it is only half of the "if and only if" condition. Y is necessary for X to happen, but we don't know if it is sufficient.

Question:
I Agree that X happens only because of Y means X--->Y, that takes care of the "only if condition". Now in this example:

It gets "chilly in summer" because of the "unexpected cold front"

Does it not mean
unexpected cold front ----> chilly in summer

All Iâ€™m doing after this point is adding an "only " clause to it

Yes the word "only" makes all the difference.

If you say X happens because of Y, then it means Y->X. It gets cold because the cold front moves in. If cold front, then it gets cold. Other things may lead to X too. When we see X it doesn't mean Y must be there. Winter makes it cold too, even without a cold front moving in. So when you feel that it is getting cold, you can't say that there must be a cold front moving in.

If you say X happens "only" because of Y, then it means X->Y. If you say that it only gets cold when there is a cold front, then whenever it gets cold, you know that there is a cold front. What you don't know from that statement though, is whether everytime when the cold front moves in it gets cold.

"if" and "only if" is the formal usage. "because" is simply common language, so you need to carefully read the sentence and translate it into the logistic languages. I do not think there is an equivalent for "if and only if" using "because", but there will be many other common languages that would express the same thing.

Question:
One more time. X is the only cause of Y. Can you throw more light on this. What does it mean and what does it not mean?

Only X causes Y, no others causes Y. When we see Y we know X must be there. Y->X.

I don't think X causes Y every time though, so we can't say X->Y.

eg. Lacking food is the only cause of starving to death. If somebody is starved we know he must be in lack of food. But if somebody lacks food he might not be starved to death.
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20 Jun 2006, 05:25
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Futuristic provided me with another good example for logic type CRs.

Futuristic wrote:
Not all tenured faculty are full professors. Therefore, although every faculty member in the linguistics department has tenure, it must be the case that not all of the faculty members in the linguistics department are full professors.

The flawed pattern of reasoning exhibited by the argument above is most similar to that exhibited by which one of the following?
(A) Although all modern office towers are climate-controlled buildings, not all office buildings are climate-controlled. Therefore, it must be the case that not all office buildings are modern office towers.
(B) All municipal hospital buildings are massive, but not all municipal hospital buildings are forbidding in appearance. Therefore, massive buildings need not present a forbidding appearance.
(C) Although some buildings designed by famous architects are not well proportioned, all government buildings are designed by famous architects. Therefore, some government buildings are not well proportioned.
(D) Not all public buildings are well designed, but some poorly designed public buildings were originally intended for private use. Therefore, the poorly designed public buildings were all originally designed for private use.
(E) Although some cathedrals are not built of stone, every cathedral is impressive. Therefore, buildings can be impressive even though they are not built of stone.

The first step I did is to symbolize the stem:
Not all T are F. All L are T. Therefore not all L are F.

The correct conclusion is that it may be true that not all L are F, but it is equally true that all L are F. If we know that all T are L, but not all T are F, then we know for sure not all L are F.

It may be helpful to think the other way. Some Ts are not F. All L are T. But all Ls may be the Ts that are F, or they may be the Ts are not F, we don't know.

So now we know what the error is, then we can proceed.

Quote:
(A) Although all modern office towers are climate-controlled buildings, not all office buildings are climate-controlled. Therefore, it must be the case that not all office buildings are modern office towers.

All M are C. Not all O are C. Therefore not all O are M.
Compare this with the stem.
All L are T. Not all T are F. Therefore not all L are F.
Note L corresponds to M and T corresponds to C. The second condition is not the same.

Quote:
(B) All municipal hospital buildings are massive, but not all municipal hospital buildings are forbidding in appearance. Therefore, massive buildings need not present a forbidding appearance.

All H are M. Not all H are F. Therefore not all H are F.
Again you see the flow is different. Both first and second condition starts with H, while in stem it flows like this: L->T->F.

Quote:
(C) Although some buildings designed by famous architects are not well proportioned, all government buildings are designed by famous architects. Therefore, some government buildings are not well proportioned.

Not all F are W, all G are F. Therefore not all G are W.
Stem:
Not all T are F. All L are T. Therefore not all L are F.
Exactly the same flow.

Quote:
(D) Not all public buildings are well designed, but some poorly designed public buildings were originally intended for private use. Therefore, the poorly designed public buildings were all originally designed for private use.

Not all P are W. Some nonW are U. Therefore all nonW are U.
Not same, don't you think?

Quote:
(E) Although some cathedrals are not built of stone, every cathedral is impressive. Therefore, buildings can be impressive even though they are not built of stone.

Some C are nonS. All C are I. Therefore some nonS can be I.
Again not the same logic.

For this type of questions it is very helpful to symbolize it, then you can ignore the contents and simply compare the logic structure. For this question you don't even need to understand the logic and find the error. Simply translation and comparation would be sufficient. However, it is often very helpful if you could understand why a logic is fause for logical questions.
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20 Jun 2006, 05:34
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Another good example where logic can be applied:

Quote:
Brown dwarfsâ€”dim red stars that are too cool to burn hydrogenâ€”are very similar in appearance to red dwarf stars, which are just hot enough to burn hydrogen. Stars, when first formed, contain substantial amounts of the element lithium. All stars but the coolest of the brown dwarfs are hot enough to destroy lithium completely by converting it to helium. Accordingly, any star found that contains no lithium is not one of these coolest brown dwarfs.
The argument depends on assuming which one of the following?
(A) None of the coolest brown dwarfs has ever been hot enough to destroy lithium.
(B) Most stars that are too cool to burn hydrogen are too cool to destroy lithium completely.
(C) Brown dwarfs that are not hot enough to destroy lithium are hot enough to destroy helium.
(D) Most stars, when first formed, contain roughly the same percentage of lithium.
(E) No stars are more similar in appearance to red dwarfs than are brown dwarfs.

Fact: BD are similar to RD in appearance but RD can burn hydrogen and BD cannot.
Fact:Stars have lithium when first formed.
Fact: All stars but the coolest BD can destroy lithium completely.
Conclusion: If there is no lithium, it is not a coolest BD.

Looking at the facts the first one doesn't do much except to tell us BD is cool. Two and three are important. The logic here is that all NonC(oolest BD) can do D(estroy lithium). Therefore if D it is nonC. Obviously the logic is wrong in saying if A is B then B is A. We know that all nonC can D, and some C cannot D, but perhaps some C can D also? Now look at the options.

Quote:
(A) None of the coolest brown dwarfs has ever been hot enough to destroy lithium.

Aha, exactly what we are looking for. Obviously it is assuming that none of the C can Destroy lithium, so that we can reach the conclusion. (If all A is B, and none B is nonA, then B is A.)

You then look at the rest of the options, and make sure there isn't something that is equally plausible. If so then you need to reread everything again.

Quote:

(B) Most stars that are too cool to burn hydrogen are too cool to destroy lithium completely.

We already know that hydrogen is irrelevent here.
Quote:
(C) Brown dwarfs that are not hot enough to destroy lithium are hot enough to destroy helium.

Obviously helium is irrelevent also.
Quote:
(D) Most stars, when first formed, contain roughly the same percentage of lithium.

This one is a little curious, but since we already formed our answer, we know this is not it. It could be the answer if the question stem is different. Say if the conclusion is that "It must be a coolest BD if there are lithium in the star." Then the right answer may very well be this one, since you would be assuming all D process should have been completed when you found the star, otherwise there may be some lithrium left undestroyed when you found a star even if it's not the coolest BD.
Quote:

(E) No stars are more similar in appearance to red dwarfs than are brown dwarfs.

Again we know this is irrelevant. It can be rather quick to go through the options if you already know what you are looking for.
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05 Sep 2006, 11:30
wow my head hurts after reviewing this material

I obviously have a while to go before going to battle with the GMAT
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09 Oct 2006, 10:48
Heh. It's perfectly normal. Logical CRs are perhaps one the hardest type questions. Just keep practising using symbols and you'll find the feel of it.
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18 Oct 2006, 02:30
Is there a way out for solving these syllogism problems with the help of Venn diagram?
I have heard it is.
Will any of the experts enlighten on this please?
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23 Oct 2006, 09:15
You could, I would think. However I think that may actually make the whole thing more complicated than it needs to be.
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09 Nov 2006, 03:52
Artemov wrote:
Is there a way out for solving these syllogism problems with the help of Venn diagram?
I have heard it is.
Will any of the experts enlighten on this please?

Positive Negative

Universal All No

Particular Some Some........not

1) Some A's are not B's means atleast 1 A is not a B

2) All A's are B's means All B's are A's

3) A positive + A positive = always is a positive conclusion

4) A positive + a negative= Always leads to a negative conclusion

5) a negative + a Negtive = leads to no conclusion

6) A universal + a universal statement always gives the followin conclusions- either universal/a particular/a no conclision

7) a universal Conclusion+particular conclusion =always gives a particular conclusion

8) a particular + a particular = no conclusion

9) All A's are B's
All B's are C's
the conclision is exists = All C's are A's and vice versa

10) Some A's are C's
Some C's are A's = a conclusion cannot be derived...................

All A's are B's means that A->B is distributed But B-> A is not distributed

Some A's are B's means A->B is not ditributed nor is B->A distributed

No A's are B's means A->B is not true and B->A is not true . So here both
A and B are Distributed

11) All A's are B's
All C's are B's
No conclusion since the middle term B is not distributed in both

For a conclision to exist the middle term needs to be distributed atleast once.................................

12) All A's are B's
All A's are c's
Conclusion is Some B's are C's or vice versa

14) All A's are B's
Some A's are C's
Some b's are C's and vice versa

13) All A's are B's
Some B's are C's
No conclusion since B is not distributed......................

15) No A's are B's
Some A's are not C's
No conclusion since both premises are negative

16) many A's are b's
All b's are not C's
conclusion is some a's are not c's and vice versa

17)All a's are b's
No b's are c's
conclusion :No a's are c's and vice versa.....................

18) All a's are b's
All b's are c's
Conclision exists as b is distributed once............All a's are c's and vice versa. Some a's are c's and vice versa is also right.......

19) All a's are b's
All c's are b's
consluaion: No conclusion since the term b is not distributed

20) All a's are b's
All a's are c's
conclusion: Some b's are c's and vice versa

21) All a's are b's
Some a's are c's
conclusion : Some b's are c's

22) All a's are b's
Some b's are c's
Conslusion: No conclusion since b is not distributed at all...........

23) Many a's are b's
No b's are c's
Conclusion: Some a's are nto c's

24)No a's are b's
some a's are not c's
conlusion: No concluaion sonce both premises are negative

All+All = All /some
All+Some = Some
All+No= No/Some not
No+Some = Some not

Hope the above mentioned funda's help

sincerely,
jyotsna
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18 Jan 2007, 01:47
Thanks HongHu for the awesome post.
And guys,these things are actually tested on the GMAT in one way or the other.
So, make sure u have a good hold of these concepts.
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27 Feb 2007, 11:46
I have personally found using variable and mapping arguments to be very effective in understanding CR questions. Can anyone point me in the direction of some books or other sources that will help me get a better understanding of this concept?

thanks
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28 Mar 2007, 21:07
It is really very helpful honghu.

thanks a lot!!!
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31 Jul 2007, 15:27
jyotsnasarabu wrote:
2) All A's are B's means All B's are A's

small mistake on this one.

http://www.gmatclub.com/phpbb/viewtopic.php?t=49756
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31 Jul 2007, 15:39
all apples are fruits.

are all fruits apples?
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14 Sep 2007, 07:03
bmwhype2 wrote:
jyotsnasarabu wrote:
2) All A's are B's means All B's are A's

small mistake on this one.

http://www.gmatclub.com/phpbb/viewtopic.php?t=49756

very helpful starting point for Logic / Critical Reasoning

http://regentsprep.org/Regents/Math/math-a.cfm
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Re: If X then Y, Help for CR [#permalink]

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03 Feb 2010, 22:51
Thanks this was useful information
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Re: If X then Y, Help for CR [#permalink]

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19 Aug 2010, 07:25
Nice post, information provided here indeed couldn't find in MGMAT CR book
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Re: If X then Y, Help for CR [#permalink]

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02 Oct 2010, 12:07
do we get such questions on gmat? I think this concept is a part of logical reasoning not part of critical reasoning.
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Re: If X then Y, Help for CR   [#permalink] 02 Oct 2010, 12:07

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