Critical Reasoning

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 necessary..
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... So option (E) , which suggests regular molting of the rattle snakes was the 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]


Pls clarify....
Thanks in advance

Tough question, but E is correct.

Argument says that 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. => One could determine the age based on number of times the snake molts.

For the argument to be true, we must assume that nothing else influences the number of molts.

Lets say the exposure to sunlight influences the number of molts. Two snakes are born on the same day, and one is places in Antarctica where sunlight is very less. And another one is taken to Middle East where there is too much sun light. If exposure to sunlight influences the number of molts, these two snakes will molt different number of times and thus will have different number of section. ie If we try determining the age by counting the number of sections in their rattle, we will say that both snakes are of different age.

So, to conclude that we can reliably count the snake's age by the number of molts, we need to assume that number of times a snake molts doesn't vary for snakes under different conditions.

Here 'exposure to sunlight' was just an example. It can be replaced with 'food availability' also. That's what E does.

Good explanation....
But I still do have a doubt...
You have said that " 'exposure to sunlight' was just an example. It can be replaced with 'food availability' also."

But why can we not REPLACE "exactly once a year" to "exactly twice a year " ? If replacements are allowed, then why not (A)?

OR "Are replacements allowed for examples alone?" Like it is with your example of sunlight...

Pls reply...
Thanks in advance

hellother wrote:Good explanation....
But I still do have a doubt...
You have said that " 'exposure to sunlight' was just an example. It can be replaced with 'food availability' also."

But why can we not REPLACE "exactly once a year" to "exactly twice a year " ? If replacements are allowed, then why not (A)?

OR "Are replacements allowed for examples alone?" Like it is with your example of sunlight...

Pls reply...
Thanks in advance

Here the assumption should be that the (all)rattlesnakes molt at a constant rate, irrespective of the conditions. Only then we will be able to find the age consistently. We need not assume the value of rate of molting (once a year, twice a year ...). It doesn't matter as long as we know it is a constant for all snakes UNDER ALL CONDITIONS.

The option E says that rate of molting when food is scarce is equal to the rate of molting when food is plentiful.

when food is scarce + when food is plentiful = all the time.

SO, basically E says that rate of molting in each rattle snake is constant under all conditions. If this is not true, then we can't determine the age consistently. Hence this assumption must be true.

I feel that A is not incorrect right away. It is partially correct. If we are to accept A (Rattlesnakes molt exactly once a year) as answer, we need to further assume that this rate of molting doesn't change under any circumstance. E gives us the complete assumption.

still the confusion persists for option (A). This answer choice says "one molt-a year" and hence it will not depend if its in cold area or scarce on food for that matter.

This is an LSAT question. It is an assumption question, however and can be examined in this GMAT context.

Remember that we are looking for the REQUIRED assumption that the argument cannot survive without. This question can allow you to try negating the answer choices to see which one is required.

You can begin this assumption question the way that you would any "normal" type of question - that is you begin by identifying the conclusion and the evidence.

The conclusion here is that "one could determine a rattlesnake's age simply from the number of sections in its rattle"

Evidence: "one new section is formed each time a rattlesnake molts"

This seems like good evidence right? If a new section is formed when a rattlesnake molts then we should certainly be able to determine the age from the rattles. What could go wrong?

Well, if you negate choice E you can see what could go wrong, IF IT IS NOT TRUE that "Rattlesnakes molt as often when food is scarce as they do when food is plentiful." Then you can see that the number of rattles may have to do with the availability of food OR with age so you cannot reliably tell the age. So choice E is correct.

Now choice A is a great STRENGTHEN answer but it is not a required assumption. You see if you ADD the fact that "Rattlesnakes molt exactly once a year" then it is a definite that you can tell the age of the snake from the rattles, but what if you take this away? What if the snakes do not molt once a year? What if they molt twice a year? You could still tell the age. You see the once a year is not required just some reliable period. So choice A is too specific.

Does that help?

Another way to do this is to summarize the argument and look for a gap:


Premise: Rattlesnake rattles are brittle and often break off.

Premise: Every time a rattlesnake molts, a new section forms on its rattle.

Conclusion: If rattlesnake rattles didn't break off, one could easily "date" a rattlesnake by counting the number of sections in each rattle.


Now let's consider the conclusion. Say I bring you a rattle and you see that the rattle has four sections. How old is the snake? You must have some sort of formula -- 4x, or x^2 - 4x + 9, or whatever, where x is the age represented by each rattle -- to convert the number of sections into an age. But whatever that formula is, it's dependent on each x representing roughly the same duration. If some other factor messes things up and slows (or accelerates) the sectioning process, you can't "date" the snake: your formula won't work.

So ( E ) is an assumption you need. Without it - and at the risk of sounding like Donald Trump - you can't determine that snake's age without a birth certificate.

Thanks a lot for the wonderful explanations ...!

David@VeritasPrep wrote:This is an LSAT question. It is an assumption question, however and can be examined in this GMAT context.

Remember that we are looking for the REQUIRED assumption that the argument cannot survive without. This question can allow you to try negating the answer choices to see which one is required.

You can begin this assumption question the way that you would any "normal" type of question - that is you begin by identifying the conclusion and the evidence.

The conclusion here is that "one could determine a rattlesnake's age simply from the number of sections in its rattle"

Evidence: "one new section is formed each time a rattlesnake molts"

This seems like good evidence right? If a new section is formed when a rattlesnake molts then we should certainly be able to determine the age from the rattles. What could go wrong?

Well, if you negate choice E you can see what could go wrong, IF IT IS NOT TRUE that "Rattlesnakes molt as often when food is scarce as they do when food is plentiful." Then you can see that the number of rattles may have to do with the availability of food OR with age so you cannot reliably tell the age. So choice E is correct.

Now choice A is a great STRENGTHEN answer but it is not a required assumption. You see if you ADD the fact that "Rattlesnakes molt exactly once a year" then it is a definite that you can tell the age of the snake from the rattles, but what if you take this away? What if the snakes do not molt once a year? What if they molt twice a year? You could still tell the age. You see the once a year is not required just some reliable period. So choice A is too specific.

Does that help?

Perfect explanation, In the argument it is also given that 'rattles are brittle and sometimes partially or completely break off'. So, if rattles can break off, it would be difficult to tell the age of snake, as unsure of number of times snake has molted the rattles.
So, I was confused with (D) also.

I love choice D on this question. You see, choice D is a perfect trap. But, choice D is also eliminated from contention by the stimulus itself. The stimulus says "So if they were not so brittle, one could reliably determine..."

That means that we are not discussing the brittleness anymore. So the conclusion is set in a hypothetical world where the rattles are not brittle. So choice D is not relevant.

I must say that this is something that is much more common on the LSAT than on the GMAT.