Contrapos: Is Princeton Rev. really confusing? Forum

[chrisnlis]

Ok, all you expert contrapos-ers:

I am completely confused by Princeton's "Cracking LSAT 2012"

On pg. 186 they show the example:

"Ann will work only if Kate works." ~K --> ~A contrapos: A --> K

Then, on pg. 187 the game gives

"B is displayed only if D is displayed" They express this as B --> D contrapos: ~D --> ~B

Now, when I looked at the example ("Ann"), I didn't think it made sense (I diagrammed it as they did "Bob"), but thought the book must be right, so I "backwarded" my original thinking...then I got to the next page, and they symbolized the same type of statement exactly how I had on the previous page, in direct contrast to what THEY said was correct on the previous page.

Now I'm all messed up.

Help? And if both pages are correct for their respective statements, I really need a good explanation why, because the 2 statements look the same to me.

Which is correct???

[woosah]

"Ann will work only if Kate works."

This is represented by

A --> K

Its contrapositive is represented by

~K --> ~A

"B is displayed only if D is displayed"

This is represented by

B --> D

Its contrapositive is represented by

~D --> ~B

"Ann will work only if Kate works." ~K --> ~A contrapos: A --> K

Where you may be confused is that in this first example, they've (oddly) decided to introduce what you would intuitively deem the "contrapositive" as the "original." And then they've made what you intuitively deem the "original" into the "contrapositive."

[chrisnlis]

Yes, that is what had me going.

So, I'm inferring (haha) that it doesn't matter which one you do first, as long as you get both parts correct?

Is it true that regardless of which one you call your original and which one you call your contrapositive, if they are both correctly stated, you're golden?

[Jeffort]

Do yourself and your eventual score a favor, get rid of that book and get something better. That PR book is sometimes referred to as LSAT 'crack' because crack is bad for you.

Seriously, the PS, Manhattan or Trainer books are miles apart better in quality and far more comprehensive.

[Christine (MLSAT)]

Yes, that is what had me going.

So, I'm inferring (haha) that it doesn't matter which one you do first, as long as you get both parts correct?

Is it true that regardless of which one you call your original and which one you call your contrapositive, if they are both correctly stated, you're golden?

The fundamental thing about conditionals and contrapositives that you need to let sink in completely is that the "original" and the "contrapositive" are actually the same exact statement from a logical perspective - they are just two different ways of expressing the same logical relationship.

This is one reason why people tend to get really confused about "unless" statements, because there are two essentially opposite ways of diagramming them. 1) unless = if not and 2) unless = then and negate the if. So, with the statement:

• Bob will eat pizza unless it rains

Method #1 gives you: if it doesn't rain, then Bob eats pizza
and method #2 gives you: if Bob doesn't eat pizza, then it rains

Both methods are perfectly valid (and boil down to personal preference), and the two results are simply the contrapositives of each other.

That being said, I can't imagine why anyone *would* diagram Ann will work only if Kate works as ~K --> ~A. It's valid, just.....weird.

[mindarmed]

Throw out that book immediately and purchase Manhattan or LSAT Trainer.

[chrisnlis]

Here's what I ordered: (if someone who has used any of these-or discarded them as trash-could critique?)

1. all 4 lsac preptest books
2. latest lsat PTs in .PDF from cambridge (to get the explanations)
3. PS LG Bible *just rec'd today
4. cracking lsat (now donating to library)
5. lsat trainer *not here yet
6. kaplan180 *not here yet
7. kaplan logic games workbook *not here yet
8. Blueprint Lsat logic games *just ordered today

I will get a logic tutor if I need to, but rather try on my own first.

[politics89]

Don't bother with Kaplan 180, I used it and it was terrible.

[Mauve.Dino]

I wouldn't recommend anything with "Kaplan" stamped on the front.

Cambridge, Manhattan, PowerScore, Blueprint, LSAT Trainer--good stuff. You probably won't even need a tutor if you use the correct materials.

Also, 7sage's free LG video explanations are golden.