LR Question 22 Explanation
QUESTION TEXT: The folktale that claims that a rattlesnake’s age can…
QUESTION TYPE: Necessary Assumption
CONCLUSION: If rattlesnakes’ tails were not so brittle then we could tell how old they were by looking at their tails.
REASONING: One new section of a tail is formed each time the rattlesnake molts.
ANALYSIS: This argument is assuming that the time between molting is equal.
___________
- This would be sufficient to prove the conclusion. But it isn’t necessary. If rattlesnakes molted exactly once every six months then we could still tell how old they were by looking at their tails.
- It isn’t necessary that the tails appear identical as long as the molting periods are constant for each species.
- It would actually be better for the argument if rattlesnakes molt at a constant pace no matter how old they are.
- The argument is talking about what would be possible if the tails weren’t brittle.
- CORRECT. Otherwise rattlesnakes in an area without food might look younger than rattlesnakes in an area with lots of food (because they molt at different speeds.)
More Resources for Necessary Assumption Questions
- Negations Article: Learn about negations on the LSAT.
- Conditional Reasoning Article: Learn about conditional statements.
- Negations Drill: Practice your negation skills.
- LR Diagrams Guide: Learn how to draw LR diagrams.
- Intro to Conditional Reasoning: Learn conditional reasoning basics.
- Intro Course lesson: This intro course lesson covers Necessary Assumption questions.
- Mastery Seminar lesson: This LR Mastery seminar lesson covers necessary assumption questions.
I am sorry but the answer choice concerning food scarcity didn’t even cross my radar as a possible answer. No where in the stimulus is the concept of food availability mentioned, or even food at all for that matter. I understand why A is wrong in terms of it being sufficient rather than necessary (I originally went with A when taking this practice section) however I still have no idea how E could possibly be correct.
Get rid of the idea that something has to be mentioned to be in an answer. This is a false limit. Something merely has to be *relevant*
Food scarcity is a fact of life. Sometimes animals in nature have more food than at other times. The argument is assuming that rattlesnakes molt on a fixed schedule.
E points out that various factors could affect rattlesnake molting schedules. E could also have said:
* Rattlesnakes molt on a regular schedule
* Rattlesnakes don’t molt more often when stressed than when not
* Rattlesnakes don’t molt more often in rainy years than in dry years
* Rattlesnakes don’t molt more often in warm years than cold years
The first version is easier to understand, but regular schedules implies that *nothing* affects the schedule: not weather, not heat….and not food scarcity.
(E) raises a question about commonsense on the LSAT. So I dismissed (E) because the negation of (E) doesn’t wreck the argument, as far as I can tell, due to the possibility that rattlesnakes could have a steady food supply that does not vary. If that were the case, (E) would not be a necessary assumption.
However, I’m assuming the LSAT folks would rebut this claim by saying it’s commonsense that food supply for an animal will not be steady in all cases. Am I correct?
Also, am I correct in saying that this is a feature of old tests more so than new? I can’t recall any NA questions from tests 70-90 that would require you to make some “commonsense leap” like this to see why a answer is correct/why a negation would destroy an argument.
Thanks in advance!!!
I’ve seen tests from all eras require common sense assumptions. Not only a feature of NA questions
Found this question because a former LSAT testmaker wrote in an interview that this was his favorite LR question of all time. If I hadn’t gone in with that knowledge, I totally would’ve fallen for A! This question is indeed really well designed–so much trickier than it seems!
Hi Graeme,
Could you please explain a little bit more about Option A?
What do you mean when you say that A is sufficient but not necessary?
This question asks us to find an assumption that is required in order for the conclusion to be true. That means we need an assumption that when negated and then plugged back into the argument, would make the argument fall apart. A sufficientassumption, on the other hand, is not necessarily necessary in order for the argument to be true. Rather, when you plug a sufficient assumption into the argument, then the argument’s conclusion follows from the premises in an airtight way — that is, it’s sufficient for the validity of the argument. There could be many potential sufficient assumptions.
Looking at (A), when we plug it back into the argument, yes, the conclusion follows from the premises. It’s a sufficient assumption. If rattlesnakes molt exactly once a year, then we could reliably determine the age from the number of sections in the rattle. BUT, this assumption isn’t necessary. If we negate it and plug it back into the argument, the argument doesn’t fall apart. Maybe rattesnakes molt exactly every six months — so we could still determine the age from the number of sections.