#3 - Joan got all A's on her homework assignments

[gwlsathelp]

Hi, how did you all set this up and answer this flawed reasoning question. The correct answer indicates that the author made an assumption. Was this something I could diagram out?

[KelseyWoods]

Hi gwlsathelp!

You could diagram this stimulus. It would look something like this:

Premise: A on term paper Pass course without presentation

Premise: A on term paper

Conclusion: Pass course without presentation

This is a Mistaken Negation. the author is saying that since Joan did not meet the sufficient condition, then she must not have met the necessary condition. But that does not follow conditional logic. If Joan had received an A on the paper she could have passed the course without doing the presentation but not receiving an A on the paper is not enough to prove that she cannot still pass the course without doing the presentation.

This is the flaw that answer choice (B) describes: "presupposes without justification that Joan’s not getting an A on her term paper prevents her from passing the course without doing the class presentation." The author assumes that the fact that Joan did not get an A on her paper is enough to prove that she cannot pass the course without doing the presentation, but this is not justified by the original conditional rule.

Hope this helps!

Best,
Kelsey

[Mastering_LSAT]

Could you please explain why you diagrammed the stimulus the way you did? Why didn't you apply the "unless equation" from Logical Reasoning Bible?

If we apply the "unless equation," then we would have diagrammed the stimulus as following:

NOT A on term paper --> Pass Course with Presentation
Contrapositive: NOT Pass Course with Presentation (i.e., without presentation) --> A on term paper

I'm confused about this. It seems to me that this question goes against standard rules that we should apply to statements with the "unless equation". Please help. Thank you!

[Ryan Twomey]

Hey Mastering LSAT,


Well there is an if statement here. So diagramming it as you did would not be correct.

The stimulus states: if Joan gets an A on her term paper then she will pass the course regardless of whether or not she does the presentation.

So you could map out that premise as this:

A term paper------>pass course

That is a fact that is undebatable based on the stimulus, so I would sit with the above for a minute instead of just applying the unless rule.

But then if you want to make the stimulus easier to comprehend, you could add this to your conditional statement without being logically incorrect:

A term paper------> pass course without presentation

This statement above is also true.

To sum up the above, when unless or without is paired with an if statement, you are probably better off not applying that trick and forcing yourself to think through it logically. In all other cases, I'm sure you are doing well with the unless trick.

I hope this helps and I wish you all the luck in your studies.

Best,
Ryan

[Mastering_LSAT]

Thanks, Ryan. Your explanation helped to clarify this issue. Are there any other instances on LSAT when we shouldn't apply the "unless equation" rule to conditional statements? Thanks again!

[Robert Carroll]

Mastering,

I think the key difference here is that the phrase is "even without" and not just "without". I think the discussion at the following link is relevant: https://blog.powerscore.com/lsat/how-to ... nt-matter/

In the present stimulus, the "even without" is contributing to make something like the following a good translation of the statement: "If she gets an A on the term paper, then doing the presentation is not necessary for passing." "Even without" is not setting up a conditional, but in fact denying that a conditional is true - if she gets an A, then the presentation is not necessary for passing. Of course, if she doesn't get an A on the term paper, we can't say that doing the presentation is necessary for passing, because that's saying that the negation of the sufficient condition leads to the negation of the necessary condition, which is just Mistaken Negation. And that's the problem here.

Robert Carroll

[Mastering_LSAT]

Thanks, Robert. Appreciate the clarification and reference to additional resources.

[kristinajohnson@berkeley.edu]

Hello,

For this flaw problem, answer D, "ignores the possibility that if Joan has to do the class presentation to pass the course, then she did not get an A on her term paper" looks like the contrapositive of the conditional premise, if A then not class presentation. I'm having trouble understanding what b says, "presupposes without justification that Joan’s not getting an A on her term paper prevents her from passing the course without doing the class presentation," this looks like if not A then not class presentation to me? But the conclusion, a mistaken negation of the premise, says if not A then class presentation, so it doesn't seem to be the correct flaw?

Can someone please explain if b is supposed to say, if not A then class presentation, how to get there? Otherwise, maybe explain where I went wrong. Thank you.

[Dana D]

Hey Kristina,

Answer choice (D) cannot be correct becuase the stimulus tells us that if Joan gets an A no need to do the presentation.

Therefore, as you correctly stated, the contrapositive would be:

need to present Joan gets an A

That's exactly what answer choice (D) says, so that's not the flaw.

On the other hand, answer choice (B) suggests that just because an A was definitely good enough to excuse Joan from the class presentation doesn't mean that she couldn't have also been excused with a lower grade. Say Joan got a B on her term paper - maybe that was also high enough to excuse her from the presentation.

Answer choice (B) points out the author's flawed logic, which incorrectly says

if Joan gets an A no need to do the presentation, so

Joan gets an A Joan needs to do the presentation.

Do you see how that's incorrect logic?

[pandapaws]

I thought that answer choice D was correct because it rightfully states the contrapositive form of the logic from the argument, demonstrating how the author overlooks this, since the author made a mistaken negation error instead of following this contrapositive form.