LR Question 24 Explanation
QUESTION TEXT: If the concrete is poured while the ground is wet…
QUESTION TYPE: Parallel Reasoning
CONCLUSION: Even settling ➞ poured while ground was dry OR crack
REASONING: Wet ➞ Solid Foundation ➞ Settle Evenly or Crack
ANALYSIS: I solved this question in 20 seconds. I just looked at the structure of the conclusion, which said “either A or B will be true”. Only one answer, A, matched that structure.
That’s the easy way, and probably how you should solve a question like this under timed conditions. But, if you’re really interested in the structure of arguments, you can follow along with the rest of this explanation to see how this is a good argument, and why the conclusion is true.
Get a pencil and paper and draw it yourself, this is easily the least intuitive argument I’ve found in logical reasoning, ever. What follows is only for advanced students.
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Let’s review the logic. The stimulus gives two conditional statements. They join on “not having a settled foundation.” I’m drawing the diagram with acronyms, look at the reasoning section above if you’re not sure what they refer to:
W ➞ SF ➞ SE or C
SE and C ➞ SF ➞ W
I’ve drawn the contrapositive as well.
Now, the question does something with logic that I’ve never seen any other LSAT question do. The conclusion is a correct deduction, but if you read the diagram left to right, you won’t see it. Let’s take a look at why the deduction is nonetheless true.
Look at the contrapositive diagram. There are two sufficient conditions: settling evenly, and not cracking. We need both to prove the necessary.
If the concrete settles evenly, there are two possibilities: either the concrete cracks, or it doesn’t:
SE and C ➞ SF ➞ W
If the concrete doesn’t crack, then we can conclude SF and W. (Not wet is the same as dry. There’s no in between with wet and dry.)
If the concrete does crack….well, that was the conclusion, right? Either the foundation is dry or it will crack. Voila.
So the two possibilities in the conclusion just describe the concrete either cracking or not cracking.
___________
- CORRECT. This conclusion matches the “either/or” structure from the stimulus. That’s really all you need to know, since every other answer fails this test.
That said, here is the diagram that shows this really matches the structure of the stimulus:
BandD➞ PE ➞ WP
Not Blurred is the same as poured evenly in the stimulus, and the two exclusive possibilities are either dark or not dark, which proves the conclusion “dark or working properly”. See the analysis above for the full explanation. - This says “both properly exposed and properly developed”. We’re looking for something that says either/or.
- This says “the camera is working properly”. We’re looking for something that says either/or.
- This says “the photograph will not be dark.” We’re looking for something that says either/or.
- This says “A or B ➞ Will not work properly”. We’re looking for something that says “A ➞ either B or C
Recap: The question begins with “If the concrete is poured while the”. It is a Parallel Reasoning question. To practice more Parallel Reasoning questions, have a look at the LSAT Questions by Type page.
More Resources for Parallel Reasoning Questions
- Conditional Reasoning Article: Learn about conditional statements.
- LR Diagrams Guide: Learn how to draw LR diagrams.
- Intro Course lesson: This intro course lesson covers Parallel Reasoning questions.
- Mastery Seminar lesson: This LR Mastery seminar lesson covers parallel reasoning questions.
In this question the structure of A) is the same as the stimulus, but the negations are not. The stimulus = SE and C –> SF –> W and the answer in A) is B and D –> PE –> WP. So my question is when I am doing these types of questions where it asks me to compare the structure, is the negation of any one element irreverent or is only relevant when deciding between two answers with the same structure that match the stimulus’ structure in which case I would go with the identical negations or is negations always irreverent?
There are a couple issues with the chains you drew.
The stimulus chain is:
W –> ~SF –> ~SE or C
Contrapositive: SE and ~C –> SF –> ~W
Here is the chain in (A):
~WP -> ~PE -> B or D
~B and ~D –> PE –> WP
We’re looking to match the overall structure of the stimulus. So, note how the premises chain together to get to the conclusion, as well as the particular kind of conclusion (in this case, an either/or conclusion.) So yes, matching overall structures take precedence over matching negations of particular terms (as we see this question). If you did come across a question where one answer choice matched the structure and each of the particular negations of the stimulus, and where the other answer choice just matched the structure, then the former of the two answer choices would be correct.