LR Question 21 Explanation
QUESTION TEXT: If the law punishes littering, then the city has an…
QUESTION TYPE: Flawed Parallel Reasoning
CONCLUSION: The city doesn’t need to provide trash cans.
REASONING: If the city punishes littering, then the city must provide trash cans. But the city doesn’t punish littering.
ANALYSIS: This argument makes a mistaken negation. The author incorrectly assumes that if a sufficient condition isn’t true, then a necessary condition isn’t true.
It’s possible that the city should provide trash cans, even if the law doesn’t punish littering. Maybe the city has an obligation to help keep the city clean.
Here’s the premise:
P ➞ TC
Here’s the incorrect negation:
P ➞ TC
You’re only allowed to negate and reverse a premise. That gets you the contrapositive.
___________
- This is an incorrect reversal. H ➞
Oturned intoO➞ H. - This is almost a good argument, but not quite. The argument has given good evidence that Jenny’s birthday party hasn’t started yet.
But it might be her birthday. The balloons could be coming later. Or maybe her birthday party is not on her birthday. - This is also an incorrect reversal. S ➞ A turns into A ➞ S.
- CORRECT. This is an incorrect negation. L ➞ M becomes
L➞M. - This is a good argument. It uses the contrapositive of the premise to prove the law isn’t being enforced.
“No one in jail ➞ law not enforced.”
More Resources for Flawed Parallel Reasoning Questions
- Conditional Reasoning Article: Learn about conditional statements.
- LR Diagrams Guide: Learn how to draw LR diagrams.
- Flaw drills: Practice identifying flaws.
- Intro Course lesson: This intro course lesson covers Flawed Parallel Reasoning questions.
- Mastery Seminar lesson: This LR Mastery seminar lesson covers flawed parallel reasoning questions.
Hi LSATHacks/Graeme. For answer B I diagrammed it like this; If balloons then birthday. No balloons so no birthday. Why would B not constitute a mistaken negation? I’m finding a hard time putting into words an answer to that question even though it feels like I know intuitively that its a good argument.
Hi! So, first, the first sentence is not If balloons –> birthday. There’s two issues with writing that as the conditional. First, the first statement uses birthday PARTY, not just birthday. This is an important distinction. So our two conditions in the first sentence are balloons and birthday PARTY. Second, the presence of balloon does not guarantee a birthday. You could have balloons for any number of reasons. The stimulus actually tells us the opposite. The first sentence says If birthday party -> balloons. The second one you diagrammed correct. So, we have:
1. If birthday party -> balloons
2. No balloons -> no birthday
So, the negation is technically correct except we don’t know that we can use birthday and birthday party interchangeably. This is what Graeme was getting at in his explanation. If there’s no balloons, we know it can’t be her party, but we don’t know it’s not her birthday. Her party might be planned for a different day (so the birthday is today but not the party, hence no balloons), or maybe the party hasn’t started yet so the balloon’s haven’t arrived but it’s still her birthday, etc. Hope that helps!