GMAT Verbal & Essays

I don't know if this is the right place where to post.
Consider this statement: "the only reason of A is B"
Is this statement equivalent to A->B or to A<->B (that means A->B and B->A)?
For example: suppose that it is true that "the only reason why an avalanche happens in a certain montain is that somebody skied in that montain on the fresh snow in a risky place"
Suppose you are told that your friend Mike just skied in that montain on the fresh snow in a risky place. Would you deduct that an avalanche happend?
Suppose you are told that an avalanche in that montain happened. Would you deduct that somebody skied in that montain on the fresh snow in a risky place?
If you would deduct only the second one then you think that "the only reason of A is B" is equivalent to A->B (as I do); if you would deduct both the first one and the second one then you think that "the only reason of A is B" is equivalent to A<->B.
I'm asking you guys because I spoke with a mathematician friend who studies mathematical logic and he says that he would deduct both the first and the second one because he thinks the statement is equivalent to A<->B
I was thinking that it is useless to continue exerciting in CR questions before I still have doubts like this.
So, guys, please help me!
Other two questions: are "the reason of A is B", "the only reason of A is B", "A is caused by B" all equivalent in your opinion?
and if I say "one of the reasons of A is B" does this mean "among all Cs such that A->C there is B"?

To me, you wording indicates that "skiing on fresh snow in a risky place" is a necessary condition for an avalanche.

If an avalanche happens, the risky skiing must have happened, but simply because the risky skiing happens does not guarantee an avalanche.

For perhaps a simpler example:

Passing the bar exam is necessary to become a lawyer. If you become a lawyer, then you must have passed the bar exam. If you passed the bar exam, you aren't necessarily a lawyer. Maybe you decided to take a different job, go back to school, etc.

Bill@VeritasPrep wrote: If an avalanche happens, the risky skiing must have happened, but simply because the risky skiing happens does not guarantee an avalanche.

Exactly! I do agree with you. But since my friend was a mathematical logic student I had some doubt on it.
What about the other two questions:

simone88 wrote: Other two questions: are "the reason of A is B", "the only reason of A is B", "A is caused by B" all equivalent in your opinion?
and if I say "one of the reasons of A is B" does this mean "among all Cs such that A->C there is B"?

simone88 wrote: What about the other two questions:
Other two questions: are "the reason of A is B", "the only reason of A is B", "A is caused by B" all equivalent in your opinion?
and if I say "one of the reasons of A is B" does this mean "among all Cs such that A->C there is B"?

Both A only if B and Only if B, A mean the following:
If A, then B.
In other words:
To have A, B is a NECESSARY condition.
A can't happen without B.

To illustrate:
John can be in Times Square ONLY IF he's in New York.
ONLY IF John is in New York can he be in Times Square.

In other words:
For John to be in Times Square, it is NECESSARY that he be in New York.
John CAN'T be in Times Square if he's NOT in New York.
The reverse, however, is NOT true:
John CAN be in New York even if he's NOT in Times Square.

GMATGuruNY wrote:
Both A only if B and Only if B, A mean the following:
If A, then B.

Yeah... I know that. But I don't understand the correlation with my questions...
simone88 wrote:
What about the other two questions:

simone88 wrote: Other two questions: are "the reason of A is B", "the only reason of A is B", "A is caused by B" all equivalent in your opinion?
and if I say "one of the reasons of A is B" does this mean "among all Cs such that A->C there is B"?

simone88 wrote:

GMATGuruNY wrote:
Both A only if B and Only if B, A mean the following:
If A, then B.

Yeah... I know that. But I don't understand the correlation with my questions...
simone88 wrote:
What about the other two questions:

simone88 wrote: Other two questions: are "the reason of A is B", "the only reason of A is B", "A is caused by B" all equivalent in your opinion?
and if I say "one of the reasons of A is B" does this mean "among all Cs such that A->C there is B"?

You're worrying about distinctions that are relevant to the LSAT but not to the GMAT.
Here is our primary concern on the GMAT:

Generally, a causal relationship offered as a PREMISE is a FACT not in dispute.
To illustrate:

The increase in the tax rate in Town X is causing residents to relocate to towns with a lower tax rate. To combat the exodus of residents from Town X, the mayor has proposed the building of a new high school.

Here, it is accepted as FACT that the increase in the tax rate is causing residents to move away.
The issue at hand is whether building a new high school will combat the exodus.

A causal relationship offered as a CONCLUSION is based upon an ASSUMPTION that CAN be disputed.
To illustrate:

Last year, the tax rate in Town X was increased by 5%. Recently, many residents of Town X have relocated to neighboring towns. Clearly, the increase in the tax rate is responsible for the exodus of residents from Town X.

Here, the CONCLUSION is that the increase in the tax rate is causing the exodus.
A conclusion is NOT a fact.
The assumption here is that there is not ANOTHER reason for the exodus.