Logical reasoning PrepTest 131 · Section 2 · Question 9

Incorrect. "Not all" is not the same as "None." "Not all" means "some don't," which is very much weaker than "none."

Incorrect. The premise about Ms. Walsh doesn't state whether she's an employee of the Tedenco Company, which it would need to state to match the logic of the stimulus.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Flawed Parallel Reasoning

Stimulus Summary:
Richardson Theater Group → not Performer OR not Administrator
Leon/Marta: Richardson Theater Group, not Administrator
Therefore: Leon and Marta are both performers

Answer Anticipation:
That first sentence establishes a conditional rule about members of the Richardson Theater Group, so there's a good chance that the argument is going to commit a conditional logic flaw. It also means that the correct answer should mirror the logic in this statement, so it's important to diagram it correctly.

"No" statements can be tricky, especially in a complex conditional (AND/OR), so let's walk through how we got the rule we did. First, when an argument says, "No X are Y," that can be rephrased as, "All X are not Y," which is how we approached this. In essence, we apply that "No" to the second term, negating it. Here, however, the second term was an AND statement, and when you negate an AND statement, it becomes OR. So when the stimulus says, "No one is both A and B," that is the same as saying, "Everyone is either not A or not B." And remember, on the LSAT, OR is inclusive—it's possible that someone in this theater group is neither A nor B.

And that ends up being important to the flaw here! Leon and Marta are members of the group, which tells us that they're either not a performer, not an administrator, or not both. However, the argument establishes that they're not administrators, and then treats that as if it establishes that they are performers. The rule, again, leaves open the possibility that they're neither, but the argument treats it as establishing that they're at least one of the two, so the correct answer should do the same.

A → not B OR not C
A and not C
Therefore: B

Answer Explanation:
Company → not HQ in Canada OR not HQ in Mexico
D/T: Company AND not HQ in Mexico
Therefore: HQ in Canada
That's the same structure and same flaw as the stimulus. These companies could have their HQs somewhere other than Mexico or Canada.

Key Takeaway:
When a flawed argument features conditional logic, it's likely that the flaw is in the application of that conditional logic. Make sure you get the conditional logic right so that you can correctly identify what type of conditional logic flaw is present!

Incorrect. This argument is valid—someone can't represent both companies, so if they represent one, they can't represent the other.

Incorrect. This argument is also valid—if there's no overlap between the boards and each has at least 5 members, we can add those numbers up to get a minimum size for the two boards.