#21 - On the Discount Phoneline, any domestic long-distance

[Administrator]

Complete Question Explanation

Parallel Reasoning. The correct answer choice is (E)

The rules of the Discount Phoneline are pretty simple: for calls that begin from 9:00 a.m. until 5:00
p.m., domestic long distance calls cost 15 cents per minute. All other domestic long distance calls
are charged at a rate of 10 cents per minute. Based on these two rules, the author concludes that, with
regard to domestic long-distance calls on the Discount Phoneline, any call that doesn’t cost 10 cents
per minute must cost 15 cents per minute.

The question stem asks for the answer choice that most closely parallels the reasoning in the
stimulus; there, the author has provided two rules: one for a certain scenario, and another for all
other scenarios (from 9 to 5, the cost is 15 cents, and it’s 10 cents for all the rest), and drawn the
valid conclusion that any call that doesn’t fall under one category falls under the other.
Answer choice (A): The reasoning in this choice looks quite similar to that of the stimulus, but
it goes off track toward the end. “If a class involved extensive lab work, it takes place in a lab;
otherwise, it takes place in a classroom”...in order to parallel the reasoning in the stimulus, this
should have ended with “Thus, if a class doesn’t take place in a lab, it must take place in a normal
classroom.”

Answer choice (B): This choice starts out correctly but doesn’t finish in the same way as the
reasoning in the stimulus. “If a class involves extensive lab work, it takes place in a lab; otherwise,
it takes place in a classroom.” In order to perfectly replicate the reasoning from the stimulus,
this one should have ended with “Thus, if a class doesn’t take place in a lab, it must take place
in a classroom.” Instead, the conclusion takes a slightly different turn and rules this choice out of
contention.

Answer choice (C): The stimulus sets out two possibilities and concludes that if something doesn’t
fall under the first category, it must fall under the second. This choice sets out two possibilities but
draws a different conclusion. This one starts with, “If a class involves extensive labwork, it takes
place in a lab’ otherwise it takes place in a classroom.” At this point, to parallel the reasoning in the
stimulus, this choice should have ended with “Thus if a class does not take place in a lab, it must
take place in a classroom.” The difference between this and what is actually offered by this answer
choice is subtle, but present, and sufficient to rule out this answer choice.

Answer choice (D): This choice begins with the right sort of premises, but draws the wrong sort of
conclusion. If a class involves significant labwork it is to take place in a lab; otherwise it must take
place in a classroom.” This is where the stimulus’ author would have concluded that if a class isn’t
held in a lab, it must be held in a classroom. Instead, this answer choice concludes that if it does
involve extensive lab work, it won’t be conducted in a normal classroom.

Answer choice (E): This is the correct answer choice, as it applies the same reasoning to
university classes as the author of the stimulus applied to domestic long distance phone calls: If such
a class requires a lot of lab work, the class will be held in a laboratory, and if it doesn’t require much
lab work, it will be held in a standard classroom. So, if a university class isn’t held in a standard
classroom, it must take place in a lab. Just like the author of the stimulus did, this choice sets out two
rules—one for some types of classes, and another for all other classes. And, like the author of the
stimulus does with phone calls, this choice draws a valid conclusion with regard to classes: all that
don’t fall under one category must fall under the other.

[srcline@noctrl.edu]

Hello,

Can someone explain why A is not the correct answer. So my reasoning was that over all this seemed to me like a if not X then Y type of logic. E to me seems the opposite.

Thankyou
Sarah

[Jonathan Evans]

Hi, Sarah,

This problem is about as formal and formulaic as the LSAT gets. I would save it for your second pass through an LR section. Then I would definitely diagram this one. Start by describing the reasoning in the stimulus:

between 8 and 5 15¢
~between 8 and 5 10¢

conclusion: ~10¢ 15¢

now abstractly:

a b
~a c

conclusion: ~c b

Now match your work to the answer choices. Answer choice A looks like this symbolized:

a b
~a c

conclusion: ~a ~b

This doesn't match. Answer choice E gives you an exact match. In Parallel the Reasoning situations, you are not only interested in matching the validity of the argument (in this case, a valid argument, as are both answer choices A and E), you also have to match the mechanics of the argument piece by piece. In strict situations such as the one above, it is prudent both to describe the situation and take a couple seconds to symbolize the structure so that you don't lose track of the details while going through the answer choices.

[deck1134]

Hi PowerScore Staff,

I have stared and diagrammed this to death, but cannot see the difference between answer choice D and answer choice E. Aren't the both "correct?" I would really appreciate any help largely because I cannot seem to get it right. Isn't it true that if a class involves extensive lab work, that it will not be conducted in a normal classroom?

Thanks

[Adam Tyson]

The toughest part of this question, Deck, is that ALL the answers are "correct" in that they all represent sound logic (if, that is, you presume that a course conducted in a lab isn't ALSO conducted in a normal classroom, which I did for the purpose of this analysis). But that's not enough for our purposes! We need it to not just be logically sound, but also exactly parallel to the structure of the stimulus.

Let's use Jonathan's abstract diagrams from earlier in this thread to show the difference. As he showed us, the stimulus is:

a b
~a c

conclusion: ~c b

Let's compare that to answer D now:

a b
~a c

conclusion: a ~c

As you can see, the premises match, but the conclusion does not. That is what goes wrong in ALL the wrong answers to this question - the premises are all the same, but the conclusions vary. So now let's look at answer E:

a b
~a c

conclusion: ~c b

It's a perfect match!

Now, because diagramming is not everyone's thing, I'll suggest another approach you might consider, also very abstract. The stimulus is saying "If you are in a certain group, then you have a particular characteristic, and if you are not in that group, you have a different characteristic. Thus, if you DON'T have the second characteristic, you must have the first characteristic." (You might notice that this is just the "word problem" version of the abstract a-b-c diagrams above.)

Answer D says "If you are in a certain group, then you have a particular characteristic, and if you are not in that group, you have a different characteristic. Thus, if you ARE IN THE FIRST GROUP (big difference!), you will NOT have the SECOND characteristic (more big difference!).

Answer E, using this "word problem" approach, comes out once again to be exactly the same as the stimulus, with a conclusion about not having the second characteristic indicating that you do have the first characteristic.

This was a tough one because every answer could look like a winner! Don't just settle for "they are both valid", but look for the same structure in the premises AND in the conclusion.