December 2010 LSAT Section 3 Question 19

In West Calverton, most pet stores sell exotic birds, and most of those that sell exotic birds also sell tropical fis...

#JW on October 17, 2019

It would not let me post this question/response to the previous thread

I totally understand the diagramming for this passage; however, the "but not" in answer choice D threw me off. My question is how do you read answer choice D in it's entirety as a person would say it when it is put into a "no" conditional statement format? I totally get the "no" conditional statement in answer choice D as saying, "If you're an independently owned pet store in West Calverton, then you do not sell tropical fish..." yet how do you read the exotic bird part of that statement when you negate "but not exotic birds"? I'm just having an issue with the language in the answer choice, specifically how "but" becomes "or" because this should be an easier question as the conditional statements in the passage are very clear. Any help on this would be really appreciated, thank you.

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SamA on October 17, 2019

Hello @Jwebb,

I think I can explain this. Let's take a look at the second sentence of the passage.

Premise: Any pet store that sells tropical fish (TF) but not exotic birds (not EB) sells gerbils (G).

TF and not EB - - - - - - - - > G

Premise: No independently owned pet store sells gerbils.
This tells us that there is one combination that is not possible: (TF and not EB).
We can express this with a contrapositive. This is where the "or" comes in.

Contrapositive: not G - - - - - - - > not TF or EB

Why is there an "or" here? Due to the lack of gerbils, we have our impossible combination: a store that sells tropical fish but not exotic birds (TF and not EB.) There are three ways to alter this combination to make it possible: either remove tropical fish (not TF), or add exotic birds (EB), or both. Do you see how this is expressed by the necessary side of my contrapositive?

This is tough, if you can understand this type of reasoning quickly on a test, then you are truly a sufficient and necessary master. However, try not to complicate things by over-diagramming. As soon as I identified the impossible combination, I had enough information to select D.