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Re: If there are sentient beings on planets outside our solar system, we [#permalink]
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Conclusion: If there are sentient beings OUTSIDE the solar system, we will not be able to determine their existence unless they are atleast as intelligent as Humans. In others words, author concludes that "intelligence atleast equal to those of humans" is a NECESSARY condition to find such sentient beings

Author cites two premises
a) Because humans will not be venturing out of the solar system in the near future.
b) Any sentient being capable of communicating should be atleast as intelligent as humans are. Hmm...so intelligence atleast equal to that of humans is a NECESSARY condition for communication (please note that it is a necessary condition not a sufficient one). If beings with intelligence => that of humans don't wish to communicate or are incapable to communicate, this premise still holds

Lets examine the answer choices:

(A) There are no sentient beings on planets in our solar system other than those on Earth.
The argument addresses sentient being outside the solar system. so this is out of scope

(B) Any beings that are at least as intelligent as humans would want to communicate with sentient beings outside their own solar systems.
This assumption is not required at all. Premise B entails that intelligence = > humans is a necessary condition. we don't require this to be a sufficient condition for the argument to hold. In other words, if an intelligent beings don't wish to communicate, it still doesn't hurt the condition that they wouldn't be able to communicate had they not been as intelligent as humans (on which the argument relies)

(C) If there is a sentient being on another planet that is as intelligent as humans are, we will not be able to send spacecraft to the being’s planet anytime in the near future.
Already stated in the stimulus.

(D) If a sentient being on another planet cannot communicate with us, then the only way to detect its existence is by sending a spacecraft to its planet.
This is it. If there is another way to communicate, then the premises of the argument (premise A and premise B) are not sufficient to lead to the conclusion.

(E) Any sentient beings on planets outside our solar system that are at least as intelligent as humans would be capable of communicating with us.
This is very similar to choice B. We don't want Premise B to be a sufficient condition to reach the conclusion. Lets negate this choice - any sentient beings...would NOT be able to communicate with us. intelligence = > humans is necessary to be able to communicate. if beings of such intelligence choose not to (as in choice B) or are not able to communicate at all (choice E) that still doesn't invalidate the Necessary condition(as stated in Premise B)
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Re: If there are sentient beings on planets outside our solar system, we [#permalink]
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I am quite new to GMAT Club but have been teaching the GMAT and LSAT since most of my students were in preschool. I hope the following is not inappropriate. As a newbie, I’m not so sure.

.............

Students need to be very careful about doing questions phrased like this, especially because it’s from an LSAT Logical Reasoning section. Specifically, LSAT preptest 62, section 4, number 18.

This question type doesn’t really exist in GMAT CR. “Really” is a weird term, I realize. But check it out...

For LSAT Logical Reasoning, this question type is known as a Sufficient Assumption, where the correct answer, when combined with the argument in the stimulus, creates a valid argument.

To paraphrase: The correct answer will guarantee/prove that the conclusion is true based on the evidence/premises. This different from a Necessary Assumption question.

Sufficient Assumption questions (unlike 95% of Necessary Assumption questions) will NOT contain a synonym for necessary (depends, relies, required). They will most often employ the phrase “if assumed”, along with “conclusion follows logically” or “enables the conclusion to be properly inferred”.

PLEASE fact check me (only OG material): for GMAT CR (unlike LSAT LR), any Sufficient Assumption question will be also be asking for a Necessary Assumption.

CR questions phrased as a Sufficient Assumption are extremely rare. If anyone ever sees such a question where the correct answer does not appear to be necessary, I would appreciate a notification. You’ll probably be wrong (heh), but perhaps I’m wrong (I’m not, though).

In other words, it’s entirely possible that a sufficient assumption will also be necessary. But this absolutely does not have to be the case.

For the GMAT, the correct answer to any assumption question will be a necessary assumption as shown by the negation test (beyond the scope of this reply).

But for the LSAT, the correct answer to many Sufficient Assumption questions will NOT be necessary (the negation test will be of no help).
....................

Fortunately, in the case of #18, the correct answer is both sufficient and necessary. But again, this is definitely not always the case.

This is why students need to be careful about doing an LSAT Logical Reasoning. Several question types that look familiar actually do not show up on GMAT Critical Reasoning.

Posted from my mobile device
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Re: If there are sentient beings on planets outside our solar system, we [#permalink]
Understanding the argument - ­
If there are sentient beings on planets outside our solar system, we will not be able to determine this anytime in the near future unless some of these beings are at least as intelligent as humans. - Conclusion. Or we can look at it as A unless B. If not B, A. If they are not as intelligent, we'll not be able to find them. 

We will not be able to send spacecraft to planets outside our solar system anytime in the near future, and any sentient being on another planet capable of communicating with us anytime in the near future would have to be at least as intelligent as we are. - Supporting premise. 
1. We'll not be able to send the aircraft outside our solar system. AND
2. Any sentient beings capable of communicating would have to be at least as intelligent as we are. 

So basically, there are two ways to detect any sentient beings on another planet
1. We send our aircraft and detect them. But we can't send that per the premises. 
2. The other sentient beings are capable of communicating with us, and if they do, then they have to be "at least" as intelligent as us. 

The argument’s conclusion can be properly inferred if which one of the following is assumed?

We need to find the missing premise or necessary/minimum condition - 

(A) There are no sentient beings on planets in our solar system other than those on Earth. - "Our solar system" is out of scope. Our scope is limited to "outside our solar system."

(B) Any beings that are at least as intelligent as humans would want to communicate with sentient beings outside their own solar systems. - This introduces the "would want to communicate" desire or will of the sentient beings as an additional criterion and missing altogether the "capability part," which is a critical part of the supporting premise. The scope of our argument is "any sentient being on another planet CAPABLE of COMMUNICATING with the US." I can say I want to have breakfast on the moon. My desire or willingness to have breakfast on the moon needs to be supported by my CAPABILITY to have breakfast on the moon. Out of scope. 

(C) If there is a sentient being on another planet that is as intelligent as humans are, we will not be able to send spacecraft to the being’s planet anytime in the near future. - It says, "We will not be able to send spacecraft to the being’s planet anytime in the near future." The assumption is the missing premise, and the premise is not re-stated as a supporting premise already mentioned in the argument. Distortion. 

(D) If a sentient being on another planet cannot communicate with us, then the only way to detect its existence is by sending a spacecraft to its planet. - This option removes the 2nd option, "their capability to communicate." So, if they cannot communicate, the only way left is for us to send the aircraft. But from the premise/fact, we know we can't send. So, this situation bolsters the conclusion that "we will not be able to determine this anytime shortly." 

(E) Any sentient beings on planets outside our solar system that are at least as intelligent as humans would be capable of communicating with us. - Weakener. If they can communicate, even after we do not send the spacecraft, they can still communicate with us. So the conclusion" we will not be able to determine this anytime shortly" is weakened. 
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Re: If there are sentient beings on planets outside our solar system, we [#permalink]
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