Grouping LG: "If X, Y, or both.." Forum

[desiballa21]

Question: On conditions that have "If X, Y, or both appear, Z also appears," what becomes the contrapositive? I know when you flip and negate, you change "or" to "and" and vice versa. But what if it's like the condition I wrote and it has both an "and" and "or?"

[breadbucket]

just take out the "or both", thus, if not z then not X and not Y. not having x or y means you cant possibly have both.

[suspicious android]

Like breadbucket was saying, "or both" in that phrase is extraneous. Unless explicitly told otherwise, always assume "or" is inclusive, that is, it already means "or both".

So you'd just get

X or Y --> Z
~Z --> ~X and ~Y

[willwash]

Question: On conditions that have "If X, Y, or both appear, Z also appears," what becomes the contrapositive? I know when you flip and negate, you change "or" to "and" and vice versa. But what if it's like the condition I wrote and it has both an "and" and "or?"

Or, to actually word it:

Rule: If X, Y, or both appear, then Z also appears

Contrapositive: if Z does not appear, then neither X nor Y appear-->because if either X or Y DID appear, then Z would HAVE to appear.

EDIT: I like to actually write these out as sentences, because the minute I see stuff like:

X,Y-->Z = -Z=-X,-Y

My brain just turns off.

[desiballa21]

A second question.. what do y'all think about in/out games? I'm having a lot of trouble with them and it's hard to move on from here. Only in my third week of LSAT prep but this is the first time I'm having significant trouble. Keeping track of all the inferences, double-arrows, and contrapositives and then heading to the games seems time-consuming. Nearly took 45 minutes on that bird game on my first time seeing it. Is there another way that you recommend to keep track of the rules or to attack the question? I've heard some people use huge chains to keep track of all the rules.

[SingleLadies]

http://lsatblog.blogspot.com/2009/08/lo ... n-out.html

Steve at the LSAT blog has a good method for mapping in and out games. Check it out.

[desiballa21]

I am actually using LSAT BLOG's study guide and did see his method for in/out games. It was the huge chain.. Does that work for every in/out game?