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 evelineliu
PowerScore Staff
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#90493
Hi there,

The rule is ~(L before F) --> L is on Saturday. The contrapositive of that is: L is NOT on Saturday --> L before F. The inference you described is the contrapositive, which is valid!

You are correct that if L is on Saturday, then the other librarian on duty must be G or Z because based on the diagram, those are the only two variables with nothing that follows. As you said, it doesn't lead to determining the placement of any other variables though.

Best,
Eveline
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 ak2024
  • Posts: 3
  • Joined: Nov 24, 2023
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#104180
I'm having trouble with the diagraming the conditional rule. i had L—f —>Lsat

and the contrapositive is Lsat —> LF

what mistake did I make?
 Adam Tyson
PowerScore Staff
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#104182
That rule has the word "unless" as a conditional trigger, ak2024:
Unless Leung is on desk duty on Saturday
If you use the "Unless Equation" that we teach at Powerscore, that means "L on Saturday" is the Necessary Condition. It goes on the right side of the arrow. meanwhile, the other condition - "Leung must be on desk duty earlier in the week than Flynn" - gets negated and becomes the Sufficient Condition and goes on the left. And the negation of L being earlier than F is, in this case, that F is before L (because they cannot be the same day as each other if they are not both on Saturday, and F is never on Saturday). Thus, you get this diagram:

F - L :arrow: LSat

The contrapositive, then, is:

LSat :arrow: L-F

As you can see, you diagrammed that the other way around, which means you may have treated "unless" as indicating a Sufficient Condition instead of a Necessary Condition.

Once you get that "unless" bit right, the rest should fall into place for you!

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