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#84424
Complete Question Explanation


Must Be True—Formal Logic. The correct answer choice is (B)

The stimulus does not contain a conclusion and can be diagramed as follows:

          SP = counties with a single political party
          P = countries with a plurality of parties
          CG = corrupt national governments
          WLG = weakness of local governments
LR1.PNG
The diagram yields two inferences:
LR2.PNG
Given that the question stem is a Must Be True, you should look for an answer choice that contains
one of the two inferences. Answer choice (B) contains the second inference and is correct.

Answer choice (A): This answer contains two errors, both of which are fatal: first, it attempts to
reverse the chain between the single party and weak local governments and second it tries to change
a most statement into an all statement.

Answer choice (B): This is the correct answer. The answer can be a bit tricky to spot since the
statement is reversible, and the wording of the answer is given from the reversed perspective. This
is a common trick used by the test makers, and you should be prepared for it when solving Formal
Logic problems.

Answer choice (C): Although this answer could be true, it does not have to be true. From the inherent
inference of the statement contained in the last sentence of the stimulus, we can conclude that some
countries with weak local governments do have corrupt governments, and thus it is possible that all
countries with weak local governments have corrupt governments.

Answer choice (D): This answer choice improperly reverses the first inference.

Answer choice (E): The stimulus does not provide information as to the total number of countries
with single political parties or a plurality of parties. Thus, the statement in this answer choice cannot
be proven true. Remember, the most and some statements in the stimulus relate to different entities,
hence no comparative inference can be drawn. The following example shows why this answer is
incorrect:
LR3.PNG
 lsatdaily
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#60712
Hello,

I got the correct answer at the end but was confused between B & D. Can someone please explain B & D , and what makes B the better AC?

Thank you in advance.
 Adam Tyson
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#60740
Glad to see you're working on the LSAT daily, lsatdaily! I'm happy to help you with this question.

The issue here is one of Formal Logic, which works a lot like Conditional Reasoning but with qualifiers like "some" and "most" instead of the absolute language of conditionals (all, every, whenever, etc). When we read that most of something has a certain characteristic, we can diagram it with an arrow, like with conditionals, but we add the qualifier to the arrow. For example, if I were to say that most of my children are in the Navy, it would look like this:

My children :most: Navy

Unlike in a conditional relationship, we can infer here that some of the things to the right of the arrow are in the group to the left of the arrow. In conditional reasoning, that would be a Mistaken Reversal, but not in Formal Logic. A "most" statement going from right to left implies a "some" statement going the other way. So, if the above statement is true, then I can say for sure that at least some people in the Navy are my children. What I cannot do, however, is turn that "most" around and read it as "most of the people in the Navy are my children." I can tell you with absolute certainty that that is NOT the case! ;-)

That's the problem with answer D in this question. While I know for sure that most of the countries with single party rule have corrupt national governments, and that all of those have weak local governments, I cannot say that most of the countries with weak local governments have a single party. I can only say that some of them do, just like my kids in the Navy.

If it helps, another way to think about it is to imagine some numbers. Let's say there are 10 countries with a single political party. Let's say 9 of them (the vast majority) have corrupt national governments, and all of those 9 must have weak local governments. Now, couldn't there be 100 total countries with weak local governments? Some of them (9) have single party rule, but maybe the rest all have multiple parties? Those numbers don't conflict with the information in the stimulus, and so it could be true that most of the ones with weak local governments do not have a single political party.

Whether you follow the formal rules of Formal Logic, or you use the "throw some numbers at it" approach, I hope this helps clear up this little issue for you, and that you find it useful as you continue your studies and practice. Good luck, keep at it daily!
 halko.sarah@gmail.com
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#88957
Hello there!

Wondering if you can explain why "the vast majority of countries that have a single political party have corrupt NGs" leverages a classic :arrow: arrow, however "BUT some countries with a plurality of parties also have corrupt NGs" is displayed with a :dbl: arrow? I diagramed both as :arrow:

Many thanks,
SH
 Robert Carroll
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#88965
Sarah,

This is something we talk about in discussions of Formal Logic. In short, a "most" statement is not reversible, while a "some" statement is. And, I want to note, neither of those is diagrammed as a conditional - note that in our diagram at the top, the "vast majority of countries that have a single political party have corrupt NGs" is diagrammed with an arrow that has an "m" above it, which is our notation for "most". That's not identical to a conditional.

A "most" statement has an order to it. Note the difference between these two statements:

Most chess grandmasters started playing when they were younger than 10.

vs

Most people who started playing chess when they were younger than 10 are chess grandmasters.

The former is, I think, true. The latter is definitely not. Because "most" statements have an order, we use a symbol that also indicates an order, the arrow. So:

chess GM :most: started under 10

is the first "most" statement I showed above.

"Some" statements are reversible. Compare:

Some lowfat foods are high in sugar.

vs.

Some foods high in sugar are lowfat.

These mean the same thing: at least one food is simultaneously high in sugar and lowfat. Because the order of the statement makes no difference, our representation is double-sided:

lowfat food :some: high in sugar

Robert Carroll
 halko.sarah@gmail.com
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#89010
Thank you, Robert, for the additional insight. Extremely helpful.

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