PT 52; Q 3 - LR: Am I doing this wrong? Forum

[jduffey]

I think I have a good grasp on the stimulus but I'm having trouble fully explaining to myself why A is correct using sufficient and necessary explanations. Any help is appreciated.

Stimulus: (My reasoning)

Joe’s car is vacuumed → K & L employees vacuum it

K & L employees vacuum Joe’s car → Joe took his car to K & L Auto

Therefore, if Joe’s car is vacuumed → Joe took his car to K & L Auto



Answer: A (My explanation)

Emily drank water from the glass this morning → Glass is wet

Emily takes her medication → Emily drinks water in the morning

Therefore, if Emily takes her medication then her glass will always be wet?


I think I'm having trouble with the verbiage in the answer, (used variables to simulate the answer structure)

"X happens only if Y happens. Since the only time Y happens is when Z happens, Z must have happened"

Does this translate to "Y --> X" and "Z --> Y"?

Thanks.

[jduffey]

C'mon smarty pants. Someone show me your intellectual superiority by helping a brother out.

[JazzOne]

"X happens only if Y happens. Since the only time Y happens is when Z happens, Z must have happened"

Does this translate to "Y --> X" and "Z --> Y"?

Thanks.

No. The phrase "only if" introduces a necessary condition. So the first sentence should be symbolized:
X->Y

The phrase "the only" introduces a sufficient condition. So the second sentence is:
Y->Z

I've been teaching LSAT for a few years, so those indicators are ingrained in my head. However, I still use my common sense to interpret the natural language. Let's think about the first sentence. X happens only if Y happens. So if Y does not happen, then X cannot happen.
~Y -> ~X That's the contrapositive of the symbol I provided above (X->Y).

Now consider the second statement. The only time Y happens is when Z happens. So IF Z does not happen, THEN Y does not happen.
~Z -> ~Y is the contrapositive of the symbol above (Y->Z).

So we can infer the following:
X->Z
~Z - ~X

[academiccricket]

I've never done this one before, so this probably isn't helpful (I don't even have PT53 yet...)

But, based on the info you've presented, I think you're over thinking this. Both answers (as I'm sure you're aware) are not flawed, so you need to look for an answer choice that is not flawed.

Answer choice A isn't flawed, but the thing that seems to make it "tricky" is a simple displacement. The second premise should be first in chronology (the assumption here is that your sufficient/necessary clauses actually match the text...like I said, I don't have PT53 so I have no idea), which makes the reasoning identical to the first answer.


Emily takes her medication → Emily drinks water in the morning

Emily drank water from the glass this morning → Glass is wet

Therefore, if Emily takes her medication --> her glass will always be wet.

----

Sorry if this just obscures the issue further. Good luck!

[jduffey]

Thanks for the help. I knew it had something to do with "only if" and "the only".

Very useful tip on sufficient vs. necessary!

[Cognoscenti]

"X happens only if Y happens. Since the only time Y happens is when Z happens, Z must have happened"

Does this translate to "Y --> X" and "Z --> Y"?

Thanks.

No. The phrase "only if" introduces a necessary condition. So the first sentence should be symbolized:
X->Y

The phrase "the only" introduces a sufficient condition. So the second sentence is:
Y->Z

I've been teaching LSAT for a few years, so those indicators are ingrained in my head. However, I still use my common sense to interpret the natural language. Let's think about the first sentence. X happens only if Y happens. So if Y does not happen, then X cannot happen.
~Y -> ~X That's the contrapositive of the symbol I provided above (X->Y).

Now consider the second statement. The only time Y happens is when Z happens. So IF Z does not happen, THEN Y does not happen.
~Z -> ~Y is the contrapositive of the symbol above (Y->Z).

So we can infer the following:
X->Z
~Z - ~X

FYI
Both "only if" and "only" refer to necessary conditions.
See LRB.

[jduffey]

No. The phrase "only if" introduces a necessary condition. So the first sentence should be symbolized:
X->Y

The phrase "the only" introduces a sufficient condition. So the second sentence is:
Y->Z

I've been teaching LSAT for a few years, so those indicators are ingrained in my head. However, I still use my common sense to interpret the natural language. Let's think about the first sentence. X happens only if Y happens. So if Y does not happen, then X cannot happen.
~Y -> ~X That's the contrapositive of the symbol I provided above (X->Y).

Now consider the second statement. The only time Y happens is when Z happens. So IF Z does not happen, THEN Y does not happen.
~Z -> ~Y is the contrapositive of the symbol above (Y->Z).

So we can infer the following:
X->Z
~Z - ~X

FYI
Both "only if" and "only" refer to necessary conditions.
See LRB.

So are you saying JazzOne is incorrect?

[Cognoscenti]

No. The phrase "only if" introduces a necessary condition. So the first sentence should be symbolized:
X->Y

The phrase "the only" introduces a sufficient condition. So the second sentence is:
Y->Z

I've been teaching LSAT for a few years, so those indicators are ingrained in my head. However, I still use my common sense to interpret the natural language. Let's think about the first sentence. X happens only if Y happens. So if Y does not happen, then X cannot happen.
~Y -> ~X That's the contrapositive of the symbol I provided above (X->Y).

Now consider the second statement. The only time Y happens is when Z happens. So IF Z does not happen, THEN Y does not happen.
~Z -> ~Y is the contrapositive of the symbol above (Y->Z).

So we can infer the following:
X->Z
~Z - ~X

FYI
Both "only if" and "only" refer to necessary conditions.
See LRB.

So are you saying JazzOne is incorrect?

I'm not sure. I don't have the question in front of me (at work...sssh).
But I am certain that his statement that "only" is a sufficient indicator is incorrect. But don't trust anyone's (anonymous) advice blindly; I suggest you look it up in LRB or any formal logic text.
If you want to PM me the Q, I'd be happy to take a look. However, I'm not sure if that's against the TLS forum rules...

[JazzOne]

No. The phrase "only if" introduces a necessary condition. So the first sentence should be symbolized:
X->Y

The phrase "the only" introduces a sufficient condition. So the second sentence is:
Y->Z

I've been teaching LSAT for a few years, so those indicators are ingrained in my head. However, I still use my common sense to interpret the natural language. Let's think about the first sentence. X happens only if Y happens. So if Y does not happen, then X cannot happen.
~Y -> ~X That's the contrapositive of the symbol I provided above (X->Y).

Now consider the second statement. The only time Y happens is when Z happens. So IF Z does not happen, THEN Y does not happen.
~Z -> ~Y is the contrapositive of the symbol above (Y->Z).

So we can infer the following:
X->Z
~Z - ~X

FYI
Both "only if" and "only" refer to necessary conditions.
See LRB.

So are you saying JazzOne is incorrect?

I'm not sure. I don't have the question in front of me (at work...sssh).
But I am certain that his statement that "only" is a sufficient indicator is incorrect. But don't trust anyone's (anonymous) advice blindly; I suggest you look it up in LRB or any formal logic text.
If you want to PM me the Q, I'd be happy to take a look. However, I'm not sure if that's against the TLS forum rules...

The phrase "only" (by itself) is indeed a necessary indicator. However, the phrase in question is "the only," which precedes a sufficient condition. I've heard some people rationalize this by saying that the phrase "the only" actually refers to the second condition in the statement and is a necessary indicator.

Example: The only time I go to bed is 10pm.
Go to bed -> 10pm

That does NOT mean that I always go to bed at 10pm. Perhaps I don't go to bed at all on certain days. The phrase "the only" precedes the sufficient condition (go to bed), but it refers to the time (the necessary condition in the statement).

That justification makes sense, but it's easier for me to remember that "the only" precedes the sufficient condition. Notice how I worded my claim: "The phrase 'the only' introduces a sufficient condition." Perhaps I should have said, "immediately precedes."

X->Y
Y->Z

Those are the correct symbols for the OP's conditionals regardless of how you want to conceptualize the indicator words.

[Cognoscenti]

This one is tricky because it doesn't allow you to depend solely on the sufficient and necessary indicator words.
Although many resources (online and in LSAT books) have this list, I'll list them again:
Sufficient: if, when, whenever, every, all, any, people who, in order to
Necessary: then, no, none, only, only if, must, required, unless*, except*, until*, without*

With that said, let's tackle the stimulus:

Whenever Joe’s car is vacuumed, the employees of K & L Auto vacuum it
K&L vacuum the car --> Joe's car is vacuumed

they are the only people who ever vacuum Joe’s car
You can note the word "only" to designate the necessary condition. This statement is the same as saying, if Joe's car is vacuumed, it was done by K&L
Joe's car is vacuumed --> K&L vacuum the car

If the employees of K & L Auto vacuumed Joe’s car, then Joe took his car to K & L Auto to be fixed.
K&L vacuum the car --> K&L fixed the car

Using the last two statements, we can validate the conclusion:
Joe's car is vacuumed --> K&L fixed the car

Now, answer A is a bit trickier since the sufficient and necessary words can't be relied on alone due to the myriad ways you can express a statement in English.
Let's try:

Emily’s water glass is wet and it would be wet only if she drank water from it this morning.
Wet glass --> Drank water in the AM
(Emily's water glass being wet is part of the conclusion)

Since the only time she drinks water in the morning is when she takes her medication
Here the word list falls apart (sort of). The words list can only be directly utilized if the statement read: When she takes her medication, she only drinks water. Then it'd be straight-forward since "when" indicates the sufficient and "only" indicates necessary. But this is a completely different idea than what the test gives.
We must realize that the "when" and "only" refer to the opposite statement. If she takes her medicine, must she also drink water? (NO). If she drinks water, must she take medicine? (YES). Thus,
Drank water in the AM --> Took medicine

The conclusion is thus:
Wet glass --> Took medicine


So, to sum it up- No, JazzOne was not incorrect, especially with his/her clarification. JazzOne's method is another way of looking (and arriving at) a valid result.
However, a statement like, "only" indicates a sufficient clause is inaccurate and can be misleading if you take it at face value. It could seriously screw someone up on test day.

Please let me know if you see any mistakes; I'm always appreciative of corrections.

[JazzOne]

This one is tricky because it doesn't allow you to depend solely on the sufficient and necessary indicator words.
Although many resources (online and in LSAT books) have this list, I'll list them again:
Sufficient: if, when, whenever, every, all, any, people who, in order to
Necessary: then, no, none, only, only if, must, required, unless*, except*, until*, without*

With that said, let's tackle the stimulus:

Whenever Joe’s car is vacuumed, the employees of K & L Auto vacuum it
K&L vacuum the car --> Joe's car is vacuumed

they are the only people who ever vacuum Joe’s car
You can note the word "only" to designate the necessary condition. This statement is the same as saying, if Joe's car is vacuumed, it was done by K&L
Joe's car is vacuumed --> K&L vacuum the car

If the employees of K & L Auto vacuumed Joe’s car, then Joe took his car to K & L Auto to be fixed.
K&L vacuum the car --> K&L fixed the car

Using the last two statements, we can validate the conclusion:
Joe's car is vacuumed --> K&L fixed the car

Now, answer A is a bit trickier since the sufficient and necessary words can't be relied on alone due to the myriad ways you can express a statement in English.
Let's try:

Emily’s water glass is wet and it would be wet only if she drank water from it this morning.
Wet glass --> Drank water in the AM
(Emily's water glass being wet is part of the conclusion)

Since the only time she drinks water in the morning is when she takes her medication
Here the word list falls apart (sort of). The words list can only be directly utilized if the statement read: When she takes her medication, she only drinks water. Then it'd be straight-forward since "when" indicates the sufficient and "only" indicates necessary. But this is a completely different idea than what the test gives.
We must realize that the "when" and "only" refer to the opposite statement. If she takes her medicine, must she also drink water? (NO). If she drinks water, must she take medicine? (YES). Thus,
Drank water in the AM --> Took medicine

The conclusion is thus:
Wet glass --> Took medicine


So, to sum it up- No, JazzOne was not incorrect, especially with his/her clarification. JazzOne's method is another way of looking (and arriving at) a valid result.
However, a statement like, "only" indicates a sufficient clause is inaccurate and can be misleading if you take it at face value. It could seriously screw someone up on test day.

Please let me know if you see any mistakes; I'm always appreciative of corrections.

Edit: My bad, I misread. You got with with the double negation: "not incorrect." Your symbols are all correct in this post.

[LSAT All Star]

You all need to be aware that LSAC actively attempts to write arguments and questions so that the memorize and/or plug-n-chug techniques do not work. You will see a few examples on every exam.

[MissLucky]

Is the second clause of the first sentence totally irrelevant to the logical chain, here? "...; they [the employees of K & L auto] are the only people who ever vacuum Joe's car." Also, is this the correct translation? (If people vacuum Joe's car --> it's the employees of K&L Auto that vacuum it)

Thanks!