preptest 24 game 2 Forum

[winnatech]

I don't understand why the answer is "C" for number 8. Am I reading the rule "L performs third only if N performs fifth" incorrectly? Because I am thinking that if N is placed fifth, shouldn't L be placed third? Any help would be great, thanks

[lebob]

yes, you're reading it wrong.


L3 --> N5
which is logically equivalent to
if NOT N5 ----> NOT L3


this does not mean that N5 ---->L3



hmmm are you new to the test? i'd reaaally work on getting this idea down solid, b/c it's central to most of the test, and the mistake you made is pretty basic. work hard man, and good luck

[winnatech]

Hi lebob,

Thanks for the reply. Yes, I just recently started prepping. So for the rule that is generally like "A does this only if B does that",the diagramming would be A->B? When I read something like that, I initially think that "Oh I have to see if B actually does that, and if he does that then I know A does this" leading me to diagram it as B->A. Any ideas on how to fix this? Thanks.

[winnatech]

Anyone have ideas of how to correct this type of thinking? Thanks.

[jnorsky]

think of it this way "A happens only if B happens" so, IF A --->B

If NOT B -----> NOT A

That is the only logical conclusion you can come too with that statement. Or you could just assume that anytime you see "only if" that it is teh same as "--->"

[winnatech]

Hi jnorsky,

Thanks. I think that's my problem in thinking. For what you wrote "A happens only if B happens" I initially would think "so I have to see what B does first to know if A happens", making me think that something has to happen to B first before I can see what would happen to A (so I would see it as B-->A, which I now know is wrong).

Perhaps if I just replace "only if" to --> I would get the correct diagramming, but it still bothers me how I would think B-->A and not A-->B. Any ideas? Thanks.

[jnorsky]

Hi jnorsky,

Thanks. I think that's my problem in thinking. For what you wrote "A happens only if B happens" I initially would think "so I have to see what B does first to know if A happens", making me think that something has to happen to B first before I can see what would happen to A (so I would see it as B-->A, which I now know is wrong).

Perhaps if I just replace "only if" to --> I would get the correct diagramming, but it still bothers me how I would think B-->A and not A-->B. Any ideas? Thanks.

You are over thinking it. This test, for all its complexity, boils down to really simple logic. Even if B happens, that does NOT mean that A will definitely happen. Think about "A only if B". Is there anything in that statement that says "If B --->A??" If it was A if and only if B, then you have a double arrow where B-->A and A--->B. But when it is "only if" then there is no reason to believe that if B happens, A MUST happen. Remember, if B then A might/might not happen. But if A then B MUST happen...so A-->B.

[winnatech]

I think I understand it a lot more now. Thanks again!

[winnatech]

Sorry to bring my topic back up, but I was looking at another game in a preptest that said:

"G cannot be prescribed if both N and U are prescribed", which is diagrammed as N + U --> NOT G

Comparing this with something like "L is third only if N performs fifth"
L3-->N5,

I was wondering why the former is N + U --> not G as opposed to NOT G --> N + U. Am I thinking this wrong because there is a different means of diagramming for "if" and "only if"?

Thanks.

[DavidYurman85]

Sorry to bring my topic back up, but I was looking at another game in a preptest that said:

"G cannot be prescribed if both N and U are prescribed", which is diagrammed as N + U --> NOT G

Comparing this with something like "L is third only if N performs fifth"
L3-->N5,

I was wondering why the former is N + U --> not G as opposed to NOT G --> N + U. Am I thinking this wrong because there is a different means of diagramming for "if" and "only if"?

Thanks.

For L3-->N5, "only if" introduces a necessry condition

For N + U-->NOT G, "if" introduces a sufficient condition

Re-read about suff/necc. conditions in the LGB and g/l !

[winnatech]

Hi,

I see. So I'm thinking, if it said "L is third if N performs fifth" instead of "L is third only if N performs fifth", would the diagramming change to N5-->L3? Thanks.

[lebob]

bingo, that's exactly right


the distinction between "only if" and "if" is key for this test

[winnatech]

Great, thanks again!