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.

MGMAT ans is E. Why? Argument is basically : only if NOT(95 degrees) will rings = reliable indicator of age.

A is dismissed as requiring to many other assumptions.
A says: TEMP(<95) && GROWTH(NOT RELEVENT) -> rings reliable
Basically says growth is a function of levels of precipitation. This suggests an alternative means of causation - this is a pretty big assumption?


E says : TEMP(<95) || (TEMP(PREDICTABLE EFFECT) && TEMP(PREDICTABLE)) -> reliable.
The number of rings is predictable when temp >95. Then you need to to know the temp records in that region for the full period. And you still assume no other mode of causation. I think assuming there is no other means of causing something is a bigger assumption to make?

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.

_________________
Emily Sledge
Instructor
ManhattanGMAT

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/pos ... html#14093

_________________
Emily Sledge
Instructor
ManhattanGMAT

Sorry got it.

No problem :-)

_________________
Emily Sledge
Instructor
ManhattanGMAT