The popular notion that a tree's age can be determined

[CD]

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.

The correct answer is E, because "The conclusion is that the rings will be a reliable measure only if the temperature never exceeds 95 degrees. This is true only if there is no way to predict how many rings would be lost when the temperature does exceed 95 degrees. (If it were possible to predict this, one might be able to assess the age of a tree using its rings even if the temperature had exceeded 95 degrees.)"

HOWEVER, wouldn't you have to know HOW MANY TIMES the temperature went above 95 degrees, in addition to the number of rings that will be lost if the temp goes above 95, to be able to correctly age the tree? In other words, if the number of rings lost were predictable and it were, say 5, you would still have to know that the temperature exceeded 95, say 3 times, to know that you need to add 15 rings to the tree to determine its age.

[rfernandez]

I agree with you. In essence you would need two pieces of information to correctly determine the tree's age: the number of rings that are lost when the temperature exceeds 95 degrees and the number of times that happens. Having said that, answer E is still correct because it points out one of these factors. It is still a viable assumption even without adding the part about knowing the number of times that the temperature exceeds 95 degrees. Even without this additional information, E is the best option among the choices.

[test009]

i think the Ans should be A.

we need to uphold the conclusion. to uphold the conclusion, we need to say that in no other way or through perspiration, new rings are not added.
...meaning... if perspiration is causing reduction then it should not cause induction of new rings at the same time....

[StaceyKoprince]

I think you might be focusing on the word "only"? Make sure you pay attention to the placement of that word. The "only" applies to the particular factor being discussed (temperature above or below 95 degrees). The "only" does not say that this is the only way to measure age via rings. In the same way, I could say "only if I memorize all the math formulas will I score well on the test." That's true, it's necessary to memorize the math formulas to do well, but it's not sufficient - I have to do other stuff as well. For the argument in question, we're saying temp is a necessary factor, much as knowing the math formulas is, but we haven't said that temp is the ONLY factor.

The author states that, yes, rings can generally be used to determine age. The author then states that there is this complication with temperature. Again, the author does not state that this is the ONLY complication - it is just one complication. And the author draws a conclusion based on this one complication.

The author's conclusion is that IF the temp doesn't exceed 95 degrees, THEN you can use the rings as a reliable measure to determine the tree's age. Corollary: if the temp does exceed 95 degrees, then you can't use the rings to measure the tree's age.

So let's turn this around to test answer A:
if the author MUST believe that A is true to arrive at his/her conclusion (which is the requirement for an assumption), what would happen if the growth of new rings IS a function of the level of precipitation? If precipitation is also a factor, that doesn't automatically mean you CANNOT use the rings as a reliable measure when the temp is below 95 - they might be or they might not be, depending upon whatever's going on with that other factor. We don't know either way, and if we don't know either way, then this doesn't HAVE to be an assumption.

In contrast, if I think that I can use the rings reliably ONLY when the temp is below 95, I must be assuming that there's no way to tell what's happening when the temp goes above 95 - there's no way to get around this complicating factor. But if, say, I knew that whenever the temp exceeds 95 degrees, exactly 2 rings peel off, then maybe I can still calculate reliably even if the temp has gone above 95.

[esledge]

This question is also discussed in the following thread. I will also link to this thread from it. Please remember to search the forum before opening a new thread on a question. Thanks!

http://www.manhattangmat.com/forums/mgm ... t3915.html

[alok.anant]

Reopening an old thread:

Though most people are confused between A and E in this answer choice, I also feel that C is a strong contender:


"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"

as the argument says that Temp should NEVER exceed 95 degrees to reliably predict the age.

Wouldn't this make option C a contender: as we are saying that the assumption is that Only 1 day of temp > 95 degrees is enough to not predict the age? (by stating that in one day of such temp trees can shed their rings)

by negating the option we get that Trees cannot shed their leaves if temp goes over 95 degrees only for 1 day

and this weakens the argument which says that temp should NEVER exceed 95 degrees for prediction to hold.

Any thoughts?

[mschwrtz]

I think that you make an excellent point.

But let's suppose that we had diagrammed this sentence, and had paraphrased the conclusion as "tree rings give reliable count ONLY IF temps don't exceed 95" and the evidence as "temps above 95 (often)---> loss of rings."

If you have a topic in the conclusion that isn't present in the supporting premises, then the assumption is likely to tie that new topic to the already established topics of the premises. I don't know that I'd argue that E identifies a more important assumption of the argument than does C, but I would argue that E is a better GMAT answer.

Once in a while you may even have as streamlined an argument as we have here--one premise with one loose end (loss of rings), and a conclusion with one loose end (unreliable count). In retrospect it seems as though we should have seen E coming, but in practice you will only very rarely anticipate an answer with that sort of precision.

[as2764]

thanks, Emily, for cross-linking the two threads with the same topic. your explanation in the other thread is great, but i found this one more interesting to put in my thoughts!

HOWEVER, wouldn't you have to know HOW MANY TIMES the temperature went above 95 degrees, in addition to the number of rings that will be lost if the temp goes above 95, to be able to correctly age the tree? In other words, if the number of rings lost were predictable and it were, say 5, you would still have to know that the temperature exceeded 95, say 3 times, to know that you need to add 15 rings to the tree to determine its age.

the direction you're thinking is good, but might be just too detailed. all one would need to know IF temp ever exceedeed 95 F, is how many TOTAL rings were lost? can i count ALL of them? and E reflects this very clearly to be IMPOSSIBLE. in fact, i couldn't find any other ASSUMPTION example on the manhattangmat's CAT that could get closer to the GMAT's OG than this one.

what you're doing is analyzing the problem like a math question by breaking it into its integral components, where in:
no. of TOTAL rings lost = (no. of rings lost/day with 95+) x (no. of days with 95+)

i could boil the 1st element down even further by saying:
no. of rings lost/day with 95+ = (no. of rings lost/hr with 95+) x (no. of hrs in a day with 95+)

and i could keep going by boiling down the 1st element to minutes, seconds, milliseconds duration. and this discussion could could lead us to questions such as "what is the minimum duration of exposure to a 95+ required for the ash to lose 1 ring?"

FORTUNATELY, the argument doesn't require one to do so. as per Emily's reversal explanation in another thread, if E were NOT true, you could PREDICT ALL the rings lost and thus, predict the tree's age even when temp went 95+, hence the CONCLUSION that <95 is the ONLY time when rings tell the age, would be destroyed.

[as2764]

Though most people are confused between A and E in this answer choice, I also feel that C is a strong contender:

"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"

as the argument says that Temp should NEVER exceed 95 degrees to reliably predict the age.

Wouldn't this make option C a contender: as we are saying that the assumption is that Only 1 day of temp > 95 degrees is enough to not predict the age? (by stating that in one day of such temp trees can shed their rings)

by negating the option we get that Trees cannot shed their leaves if temp goes over 95 degrees only for 1 day

your negation is improper and that's why you're confused.
(C) says: One day of temperatures above 95 degrees Fahrenheit will cause the Brazilian ash to lose one ring.
NEGATION of (C) should say: One day of temperatures above 95 degrees Fahrenheit will NOT cause the Brazilian ash to lose one ring.

SO, the negation says one day of temperature with 95+ will not cause the ash to lose A ring. this NEGATION implies that one day with 95+ temperature will NOT cause a loss of ONE ring, leaving us confused whether how many rings (>1) would be lost during a day's worth of exposure to 95+? OR if NO rings would be lost at all? the ONLY thing we know for sure is that we will NOT lose ONE ring, but could lose NONE or GREATER THAN 1.

and this weakens the argument which says that temp should NEVER exceed 95 degrees for prediction to hold.

Any thoughts?

IN FACT, this NEGATION STRENGTHENS the conclusion that one day with a temp of 95+ could make it very UNPREDICTABLE to determine the # of rings lost and hence, the age of the tree!

this implies that (C) is actually a WEAKENER, because the NEGATION of (C) strengthens while (C) says that there is a deterministic way to count the rings lost i.e.
ONE DAY WITH 95+ = 1 RING LOST

(C) does not give us a CONCRETE method to determine the tree's age when temp goes above 95+, as @CD asked above, but it makes the determination MORE LIKELY, making the conclusion LESS LIKELY.

[jnelson0612]

Thank you ashish. I agree with your analysis and explanation.

[as2764]

thanks, Jamie. this problem took me for a roller-coaster ride, so thought it to be worthwhile to tear the argument apart into its integral components!

[jnelson0612]

Yes, you did a great job with that! Thanks again.