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The popular notion that a tree's age can be determined by counting the number of internal rings in its trunk is generally true. However, to help regulate the internal temperature of the tree, the outermost layers of wood of the Brazilian ash often peel away when the temperature exceeds 95 degrees Fahrenheit, leaving the tree with fewer rings than it would otherwise have. So only if the temperature in the Brazilian ash's environment never exceeds 95 degrees Fahrenheit will its rings be a reliable measure of the tree's age.

Which of the following is an assumption on which the argument above depends?

A. The growth of new rings in a tree is not a function of levels of precipitation. - out of scope to some extent as we are not given any information on precipitation. INCORRECT B. Only the Brazilian ash loses rings because of excessive heat. - So what?? Anyways we are supposed to talk about brazilian ash tree only. INCORRECT C. Only one day of temperatures above 95 degrees Fahrenheit is needed to cause the Brazilian ash to lose a ring.- again I would ask the same question - so what?? We are not concerned about for how much time certain temperature must remain rather we are concerned about the relationship between the temperature and the loss of rings. INCORRECT D. The internal rings of all trees are of uniform thickness. - again out of scope. INCORRECT E. The number of rings that will be lost when the temperature exceeds 95 degrees Fahrenheit is not predictable - An assumption is something which makes an argument complete. If the assumption were absent, the argument will fall apart. Here the conclusion is that only if temperature never exceeds 95 degrees, the age of tree will be predictable. What if the temperature also increases and still we are able to predict the age. The conclusion will not hold true in that case. Hence CORRECT

OA to come

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The popular notion that a tree's age can be determined by counting the number of internal rings in its trunk is generally true. However, to help regulate the internal temperature of the tree, the outermost layers of wood of the Brazilian ash often peel away when the temperature exceeds 95 degrees Fahrenheit, leaving the tree with fewer rings than it would otherwise have. So only if the temperature in the Brazilian ash's environment never exceeds 95 degrees Fahrenheit will its rings be a reliable measure of the tree's age.

Which of the following is an assumption on which the argument above depends?

A. The growth of new rings in a tree is not a function of levels of precipitation. B. Only the Brazilian ash loses rings because of excessive heat. C. Only one day of temperatures above 95 degrees Fahrenheit is needed to cause the Brazilian ash to lose a ring. D. The internal rings of all trees are of uniform thickness. E. The number of rings that will be lost when the temperature exceeds 95 degrees Fahrenheit is not predictable

Round 1 A - This can be right B - This is being clearly stated in the argument and is therefore not an assumption C - This can be right D - This is not at al relevant to the premise or the conclusion E - This has also been stated in the argument and should therefore not be an assumption

Round 2 We are left with A and C. "A" - "A" is more appropriate because it affects the conclusion directly, if this is the assumption only then conclusion can be true. "C" - for how long the temprature should stay at 95 for the ring to shed is no where being mentioned either in the premise or in conclusion..it therefore seems un-related.

Well,

I think assumption is always not that easy, because, unlike strengthening questions, assumptions seldom uses outside info.

Also, negation is not a fool-proof process, because, even negating choices that are outside scope can be misleading. For ex, we are concerned only about the heating aspect here, and its effect on the no. of rings. However, negating A will also imply that, number of rings is not predictable(since precipitation may increase no. of rings when you negate A). However, melting is beyond scope here and assumptions clarify the gap between prem. and concl. only. Thanks, Raghu

Re: The popular notion that a tree's age can be determined by [#permalink]

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26 Mar 2012, 18:17

+1 E

If it is possible to determine how many rings are lost after an increase in the temperature, it would be possible to determine the age of the tree. _________________

"Life’s battle doesn’t always go to stronger or faster men; but sooner or later the man who wins is the one who thinks he can."

Conclusion: “ONLY if the temp in the Brazilian ash’s environment NEVER exceeds 95 degree F will its rings be a RELIABLE measure of the tree’s age.”

If we truly depend on a certain assumption to reach this conclusion, the conclusion should fall apart when we remove or negate the assumption.

Negate (A): The growth of new rings IS a function of precipitation levels. Result: We could still conclude that the temp needs to stay below 95 degrees in order to use the rings as a measure. Thus, (A) is not an assumption upon which we really rely.

As you note, (A) is an appealing answer: It sure would be nice to know that there are not even more factors affecting the growth of rings. But there is a big difference between a nice-to-know assumption and an assumption-on-which-the-argument-depends. The GMAT always wants the latter!

Negate (E): The number of rings lost when temp exceeds 95 degrees IS predictable. Result: We can no longer assert that the ONLY way to use the rings to reliably determine age is if the temps stay below 95. You are correct that we would need to know the complete temperature records and number of rings lost as a function of temp, but it would at least be possible to use the rings to determine age. The “only” in the conclusion is really the key word.

Re: The popular notion that a tree's age can be determined by [#permalink]

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09 May 2012, 02:50

at first was confused with A and E... as AThe growth of new rings in a tree is not a function of levels of precipitation. B Only the Brazilian ash loses rings because of excessive heat. --not relevant...with conclusion.. C Only one day of temperatures above 95 degrees Fahrenheit is needed to cause the Brazilian ash to lose a ring. --again irrelevant.... D The internal rings of all trees are of uniform thickness. --out of scope... E The number of rings that will be lost when the temperature exceeds 95 degrees Fahrenheit is not predictable.

but ..negation of E " The number of rings that will be lost when the temperature exceeds 95 degrees Fahrenheit is predictable." hurts the conclusion ..therefore i went with E...

Re: The popular notion that a tree's age can be determined by [#permalink]

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23 May 2012, 07:49

1

This post received KUDOS

The central point of the passage is to determine the age of the Brazilian ash trees.

Taking Option E - The number of rings that will be lost when the temperature exceeds 95 degrees Fahrenheit is not predictable

If we negate the above statement, we can rephrase it as follows:

The number of rings that will be lost when the temperature exceeds 95 degrees Fahrenheit is predictable.

Now if in a year, the temperature has exceeded 95 degrees, then we would know the number of rings the tree would lose before hand. If we would like to determine the age of the tree, then all we need to do is add the number of rings lost to the existing rings.

I hope my explanation makes sense. Would love to hear a different version which would help understand the problem in depth.

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22 Feb 2015, 22:40

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10 Apr 2016, 05:32

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