November 2019 LSAT Section 1 Question 12

Which one of the following, if substituted for the condition that if jalapeño is an ingredient, yam must not be an in...

MrLaw on April 28, 2020

Why B?

When I wrote out the pure logic for A I got: J --> P ~p --> ~J Which I combined with the logic in the stimulus: P --> ~Y Y --> ~P To conclude: J --> ~Y Which is the same variable they asked us to remove. So why is B correct and not A? Thanks!

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Skylar on May 25, 2020

@Alexander-Blankers, happy to help!

You seem to be confusing the diagrams of (A) and (B). Remember, the term "only if" introduces the Necessary condition.

(A) "Potato is an ingredient only if jalapeno is an ingredient."
P -> J
NOT J -> NOT P

(B) "Jalapeno is an ingredient only if potato is an ingredient."
J -> P
NOT P -> NOT J

Therefore, the diagram that you thought belonged to (A) actually belongs to (B). The rest of your logic shows why (B) would then be the correct choice.

Does that make sense? Please let us know if you have any other questions!

Livia on August 22, 2022

Hi! I am a bit confused on why the answer would not be C.

it states ' Which one of the following, if substituted for the condition that if jalapeño is an ingredient, yam must not be an ingredient, would have the same effect in determining the addition of vegetables to the pot?'. Which says IF J then not Y, correct? Why would the correct answer be B, where it says if and only if, and not C where it says If then?

Emil-Kunkin on September 22, 2022

Hi, our initial rule is
If J -> not Y.
Its contrapositive is
If Y -> Not J

C tells us If not J -> Y
Or If not Y -> J

This is not our initial rule.

B tells us that
If J ->P, since only if introduces a necessary condition.

Since we know that if P not Y, this new rule means that if J -> Not Y.