Q9

[alexandru_p22]

This might sound like a stupid question but here goes...

For Number 9, I picked A, but this is wrong. The answer is supposedly C. Here is my diagram/thought process (not laws are in red):

H V K I N
M T W Th F
N
K

I believe this to be the only possible arrangement because, since we're given K for Wednesday, this means that we're supposed to have V on Tuesday. Furthermore, Rule 3 stipulates that when N is on Tuesday, I is on Monday. Since N cannot be on Tuesday, I cannot be on Monday, and so we have to place H on Monday, forcing us to place N on Friday (rule 2) leaving us with I on Thursday. Can someone please tell me where the fault lies in my logic? I would appreciate it.

[giladedelman]

Thanks for posting! There are no stupid questions.

Your mistake is precisely this: we know that if N is on Tuesday, I is on Monday. This DOES NOT MEAN that if N is not on Tuesday, I is not on Monday!!! That's an illegal negation! The only valid inference we can draw from a conditional statement is its contrapositive; in this case, it's that if I is not on Monday, N is not on Tuesday.

Once you recognize that, you'll agree that there are actually two possibilities here: either H or I can be on Monday. I thought it was easiest to try both cases:

H V K I N
M T W Th F

I V K H/N N/H
M T W Th F

So (A) could not be true: I can be either Monday or Thursday.

(C) is the only thing that could be true: N could come before H (or vice versa) in our second scenario.

Does that answer your question?

[alexandru_p22]

It does. Thank you.