December 2010 LSAT Section 4 Question 7

Which one of the following could be the color combinations of the glass in the three windows?

DanielM on December 11, 2020

Contrapostive of last rule

Still confused on how the contrapostive of this makes sense? "If a window does not contain purple glass, then that window contains orange glass." Isn't it possible that the Glass contains no orange and no purple? I understand this would be impossible based on the other rules. Namely that y,g,r can't be together. But how does that make sense on its own? Thanks

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shunhe on January 7, 2021

Hi @DanielM,

Thanks for the question! Well, take a look at that conditional you gave us. We can diagram that

~P —> O

And we can then take the contrapositive of this statement to get

~O —> P

which means that if there’s no orange, then there has to be purple glass. In other words, there has to be at least one of the two. So we cannot have a glass with no orange and no purple. That’s based solely off of this rule.

Hope this helps! Feel free to ask any other questions that you might have.