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I’ve come upon such a question of data sufficiency, perhaps it's mentioned already:
Is rs=rx-2?
1. r is an odd number.
2. x=s+2
The answer says that with (2), the equation could be restated as rs=rs+2r-2; therefore, 2=2r, and r=1. If r=1, then the equation above can be simplified to s=x-2, which is the same as (2). So (2) is sufficient.
This reasoning has puzzled me a lot.
Let’s say I want to know whether it’s synonym to be human and able to reason(Is rs=rx-2?), with the only data that a cat Tom is not human(x=s+2). If the hypothesis is true, Tom can’t reason(r=1).
Supposing I know for a fact that Tom can’t reason, and that to be human is to be reasoning and vice versa, I could safely deduce that Tom isn’t human, which is eureka, known to be true.
Tom or no Tom, this question is unresolved.
In more abstract terms, if p and q gives r, while p and r gives q, the truth of p and r doesn’t stand or fall with that of q, does it? Though I don’t really know why, it’s something about logic structure.
[Reveal] Spoiler: OA
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CassandraBrutta wrote:
I’ve come upon such a question of data sufficiency, perhaps it's mentioned already:
Is rs=rx-2?
1. r is an odd number.
2. x=s+2
The answer says that with (2), the equation could be restated as rs=rs+2r-2; therefore, 2=2r, and r=1. If r=1, then the equation above can be simplified to s=x-2, which is the same as (2). So (2) is sufficient.
This reasoning has puzzled me a lot.
Let’s say I want to know whether it’s synonym to be human and able to reason(Is rs=rx-2?), with the only data that a cat Tom is not human(x=s+2). If the hypothesis is true, Tom can’t reason(r=1).
Supposing I know for a fact that Tom can’t reason, and that to be human is to be reasoning and vice versa, I could safely deduce that Tom isn’t human, which is eureka, known to be true.
Tom or no Tom, this question is unresolved.
In more abstract terms, if p and q gives r, while p and r gives q, the truth of p and r doesn’t stand or fall with that of q, does it? Though I don’t really know why, it’s something about logic structure.

you are thinking too much. Answer should be "B"
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This question cannot be an official GMAT question. You are thinking true. r can be anything. You cannot assume r as 1 in DS questions.
Intern
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You are forgetting the nature of DS questions.

DS questions give you 2 pieces of true information and ask if they are enough to solve the problem.

so 2. tells you that x = s + 2. you can then rely on this piece of information to solve the main question.

So once you figure out that r=1 the question becomes
is s = x -2?
and you have the information that x = s + 2 so this is SUFFICIENT.
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Having slept on my response to this question, I have changed my mind (I was too tired to be posting last night!) You are, in my opinion, correct in your observation that the logic doesn't flow the way that I described.

The question is not asking, what is the value of r given that rs=rx-2 (in which case 2. would be sufficient). The actual question is whether or not rs=rx-2? in which case, the data is insufficient without some indication of the value of r.
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