LSAT 113 | Section 2 | Logical Reasoning: Q3

LR Question 3 Explanation

QUESTION TEXT: Joan got all A’s on her homework assignments…

QUESTION TYPE: Flawed Reasoning

CONCLUSION: Joan will have to do the class presentation to pass.

REASONING: If she had gotten an A on her paper, she could have passed even without the presentation. But Joan didn’t get an A on the paper.

ANALYSIS: This is an incorrect negation. We had: Get an A ➞ Doesn’t need presentation.

The argument incorrectly switches this to: A ➞ Does need presentation.

But it’s possible Joan wouldn’t need to do the presentation even though she didn’t get an A on her paper. Joan might otherwise have had excellent grades, and a B+ on the paper.

___________

1. Actually, the argument did consider this possibility: but it’s a mistake! It doesn’t have to be the case that Joan needed one or the other. 
2. CORRECT. We have nothing to tell us that not getting an A is a sufficient condition for having to do the class presentation. 
3. The argument wasn’t concerned with the overall grade. It only considered whether or not Joan could pass.
4. This is the same thing as A. The argument didn’t ignore this possibility: it assumes that this is true, but it has no evidence for the claim. 
5. Other students are irrelevant. This is only about Joan.