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Assumption question doubt 2

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Assumption question doubt 2

by hellother » Fri Aug 31, 2012 12:50 am
The folktale that claims that a rattlesnake's age can be determined from the number of sections in its rattles is false, but only because the rattles are brittle and sometimes partially or completly break off. So if they were not so brittle, one could reliably determine a rattlesnake's age simply from the number of sections in its rattle, because one new section is formed each time a rattlesnake molts.

Which one of the following is an assumption the argument requires in order for its conclusion to be properly drawn?

A) Rattlesnakes molt exactly once a year.
B) The rattles of rattlesnakes of different species are identical in appearance.
C) Rattlesnakes molt more frequently when young than when they were old.
D) The brittleness of a rattlesnake's rattle is not correlated with the length of rattlesnake's life.
E) Rattlesnakes molt as often when food is scarce as they do when food is plentiful.

The Official ans is [spoiler](E)[/spoiler].

[spoiler]In this question, we need to find the correlation between the molting of rattlesnakes and age...
Now opt (A) was eliminated because rattlesnakes CAN MOLT even TWICE a year and we , still , will be able to DETERMINE the age by counting the number of sections... In other words, this assumption is not absolutely necessary for the conclusion to follow..
We just must be able to show that the molting is regular and not random...because if it were random, then we cannot determine the CORRECT AGE of different rattlesnakes... So option (E) , which suggests regular molting of the rattle snakes was the correct answer... But is this NECESSARILY TRUE ? I mean ,just like (A) , there could be other possibilities like " Rattlesnakes molt as often when[Something else]is scarce as they do when [something else] is plentiful. (Going by the same logic used in (A) )...[/spoiler]. [spoiler][something else] instead of "food" given in (E) ...[/spoiler]

So please clarify when to look at the answer choices as "NECESSARARILY TRUE" and when not to ..


Pls clarify....
Thanks in advance
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by MarkSullivan » Mon Dec 03, 2012 11:52 am
hellother wrote:The folktale that claims that a rattlesnake's age can be determined from the number of sections in its rattles is false, but only because the rattles are brittle and sometimes partially or completly break off. So if they were not so brittle, one could reliably determine a rattlesnake's age simply from the number of sections in its rattle, because one new section is formed each time a rattlesnake molts.

Which one of the following is an assumption the argument requires in order for its conclusion to be properly drawn?

A) Rattlesnakes molt exactly once a year.
B) The rattles of rattlesnakes of different species are identical in appearance.
C) Rattlesnakes molt more frequently when young than when they were old.
D) The brittleness of a rattlesnake's rattle is not correlated with the length of rattlesnake's life.
E) Rattlesnakes molt as often when food is scarce as they do when food is plentiful.

The Official ans is [spoiler](E)[/spoiler].

[spoiler]In this question, we need to find the correlation between the molting of rattlesnakes and age...
Now opt (A) was eliminated because rattlesnakes CAN MOLT even TWICE a year and we , still , will be able to DETERMINE the age by counting the number of sections... In other words, this assumption is not absolutely necessary for the conclusion to follow..
We just must be able to show that the molting is regular and not random...because if it were random, then we cannot determine the CORRECT AGE of different rattlesnakes... So option (E) , which suggests regular molting of the rattle snakes was the correct answer... But is this NECESSARILY TRUE ? I mean ,just like (A) , there could be other possibilities like " Rattlesnakes molt as often when[Something else]is scarce as they do when [something else] is plentiful. (Going by the same logic used in (A) )...[/spoiler]. [spoiler][something else] instead of "food" given in (E) ...[/spoiler]

So please clarify when to look at the answer choices as "NECESSARARILY TRUE" and when not to ..


Pls clarify....
Thanks in advance
Hello there, hellother!

I think your question relates to a fairly common point of confusion on CR: the difference between a Strengthener and an Assumption.

Both should help the argument, but an Assumption is also necessary for the argument to stand. Usually, a Strengthener is just a more extreme version of one of the Assumptions, so it's easy to conflate the two.

Here's a simple example.
Company X will be putting in place several cost-cutting measures over the next fiscal year. Therefore, Company X will be more profitable.
What's the logic gap here? The conclusion is about profit, but the premise only addresses costs, not revenues! So, consider the following options:

(A) Company X's revenues will triple over the next fiscal year.
(B) Company X's revenues will not decrease substantially over the next fiscal year.

Both choices help the argument, but one is an Assumption while the other is a Strengthener. The conclusion could still hold even if choice (A) were false (perhaps revenue only doubles), but if choice (B) were false (revenues do decrease substantially) then the entire argument would be questionable and the conclusion far from certain. Therefore, (A) is a Strengthener and (B) is an Assumption.

So, do we take choices as "necessarily true"? On Assumptions questions, sometimes yes, sometimes no! The point is that an Assumption when considered true should help the argument, but when considered false should destroy the argument. This is the fundamental idea behind the "Negation Test" for Assumptions (discussed in great detail in the Manhattan GMAT CR Guide).

Back to the rattlesnakes (I hope I never have reason to type those words again!), we should first get down to (A) and (E) because those are the only choices that help the argument. Now, what happens if we negate?

In choice (A), not much. So what if rattlesnakes don't molt exactly once per year? If the molting follows some other regular pattern (say, twice per year), we can still measure age. The alternatives we generate here are only to show that the conclusion could still stand even if (A) is false. Yes, choice (A) is helpful (it actually makes the conclusion quite certain), but it is not necessary. Therefore (A) is a Strengthener, not an Assumption.

When we negate choice (E), however, the argument falls apart. If the molting frequency depends on abundance of food, then it doesn't follow *time* and therefore doesn't tell us age! As soon as molting is linked to something other than time, the conclusion falls.

So, in your example:
... just like (A) , there could be other possibilities like "Rattlesnakes molt as often when[Something else]is scarce as they do when [something else] is plentiful."
every different "something else" would actually give us another Assumption. Remember that the correct answer to an Assumptions question doesn't have to fix an argument's every problem. It just has to be one statement that is required for the argument to stand.

Assumptions = necessary, but not necessarily sufficient!

At the end of the day, CR isn't really about what's true or false - in fact, you should check your outside knowledge at your test center locker! It's about the internal structure of each argument.

Hope that clarifies things!

Best,
Mark
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by GMATGuruNY » Mon Dec 03, 2012 6:35 pm
hellother wrote:The folktale that claims that a rattlesnake's age can be determined from the number of sections in its rattles is false, but only because the rattles are brittle and sometimes partially or completly break off. So if they were not so brittle, one could reliably determine a rattlesnake's age simply from the number of sections in its rattle, because one new section is formed each time a rattlesnake molts.

Which one of the following is an assumption the argument requires in order for its conclusion to be properly drawn?

A) Rattlesnakes molt exactly once a year.
B) The rattles of rattlesnakes of different species are identical in appearance.
C) Rattlesnakes molt more frequently when young than when they were old.
D) The brittleness of a rattlesnake's rattle is not correlated with the length of rattlesnake's life.
E) Rattlesnakes molt as often when food is scarce as they do when food is plentiful.
This passage links the number of MOLTS to the AGE of the rattlesnake.
The assumption is WHAT MUST BE TRUE for this link to be valid.

Hellother is debating between A and E.
It can be helpful to preface each answer choice as follows:
For the conclusion to be valid, IT MUST BE TRUE THAT [insert answer choice].

Answer choice A:
For the number of MOLTS to be linked to the AGE of the rattlesnake, IT MUST BE TRUE that rattlesnakes molt exactly once a year.
No.
If rattlesnakes molt exactly twice a year, the age of a rattlesnake can still be determined from the number of molts.

Answer choice E:
For the number of MOLTS to be linked to the AGE of the rattlesnake, IT MUST BE TRUE that rattlesnakes molt as often when food is scarce as they do when food is plentiful.
Yes.
If the number of molts can vary with the amount of food available, then the age of a rattlesnake cannot be determined from the number of molts.

The correct answer is E.
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by hellother » Tue Dec 04, 2012 9:07 am
Hello there,

Thanks for the replies.. I found both the replies awesome and very helpful !!!
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