Argument completion and specific premise exercises

Hi! When doing practice problems for both argument completion and specific premise questions in the drill sections following this video I often end up with the contrapositive of the correct answer... why is that? I apologize if my question is vague, but I am not sure what I am missing, perhaps a step or a rule?

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DaveGreenstein December 30, 2020

For instance, looking for the missing premise of he problem:
P: A -> C
P: not Z -> A
P: ?
-----------
C: not B -> C

Why is the answer Z->B and not; not B -> not Z?

shunhe January 4, 2021

Hi @DaveGreenstein,

Thanks for the question! So actually, getting the contrapositive is fine, and correct! You can think of them as two correct, alternative answers, and we just put one of them down. Remember that contrapositive statements are logically equivalent, and so since they mean the same thing, putting one is the same thing as putting the other. So if you’re getting the contrapositive, you’re doing great!

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