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December 2004 LSAT Section 4 Question 13

Which one of the following could be a complete and accurate list of the friends who appear together in a photograph?

DiegoC on November 30, 2021

how can Y and R be in the same group if Y and R cannot be together? this is very confusing

how can Y and R be in the same group if Y and R cannot be together? this is very confusing

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DiegoC on November 30, 2021

"Raimundo appears in every photograph that Yakira does not appear in."

DiegoC on November 30, 2021

doesn't that mean that they cannot be together?

Jay-Etter on January 17, 2022

Hi Diego, here we are dealing with the topic of conditional statements, and necessary and sufficient conditions. When we say that R appears in every photograph that Y does not appear in, that means when Y is not there then R must be there. However when Y is there, then we don't know anything about R. R could appear or it could not. This means they could go together. We would write this rule as
Not Y -> R,
and its contrapositive as Not R -> Y