I'm not following this. This is the clue that I don't understand the intricacies of yet and I'm sure the answer lies in this post. But if someone could explain it in a different way it'd be much appreciated.
Scratch that, I just found this post which explains it rather nicely.
Quote:
Originally Posted by admin
Hi Stan 
This follows the very same logic from your first post at the top of the thread.
"Rare book" and "Tea Set" are both in the same category (not sure what that category is, as I'm going from memory, but let's say they are "Gifts" for the sake of this explanation).
Clue #3 says that Oakdale is either Greg's gift or the tea set, and $7.75 is either Greg's gift or the teaset. If you know that Greg's gift isn't the rare book, then you know Oakdale isn't the rare book, and $7.75 isn't the rare book.
Here's another way to look at it  the two logic statements we're using are (color coded by category):
1. Of Oakdale and $7.75, one is the tea set and the other is Greg.
2. Greg does not equal the rare book.
Take #2 and use it to replace "Greg" in #1, like so:
 Of Oakdale and $7.75, one is the tea set and the other does not equal the rare book.
We inherently know, because tea set and rare book are different items in the same category, that:
3. Tea set is not equal to the rare book.
... so now let's replace "tea set" in our modified Clue #3:
 Of Oakdale and $7.75, one does not equal the rare book (originally: Tea Set) and the other does not equal the rare book (originally: Greg).
Now the logic should be clear  none of the two options given for Oakdale and $7.75 (i.e. Tea Set and Greg) can be equal to rare book. Therefore, since Oakdale must be either "tea set" or "Greg", and $7.75 must be either "Tea Set or "Greg", Oakdale does not equal rare book, and $7.75 does not equal rare book.
