Help with LR! PT 43 - Sect 3 - Question 9

[vandyprincess]

After all of my charting and diagramming, I got D instead of A (A is the right answer) and I've spent an hour staring at it trying to figure out why. Please help. 

[ohhhyeah]

Here is the link:

All effective teachers are good communicators.

Most effective teachers are eccentric (hence, Some effective teachers are eccentric)

Eccentric <--S--> Effective teachers --> Good communicators

which leads to..

Eccentric <--S--> Good Communicators

Therefore, Some good communicators are eccentric.

[LSAT All Star]

After all of my charting and diagramming, I got D instead of A (A is the right answer) and I've spent an hour staring at it trying to figure out why. Please help. 

Here's how I do questions like this:

Before you can know what logically follows, you must first know what is true:

Most A (lecturers) who are B (effective teachers) are C (eccentric).  [Or, most (A and B) are C].
Some A who are Not C are B.  [Or, some (A and Not C) are B].
All B are D (good communicators).

Next, there can be a lot of things that follow logically.  Therefore, it does not seem like a good use of time to try to pre-phrase the correct response.  But, when I go through the response options, I will look for two things.  First, I have to know something about the premise in a response option before it can be true.  Second, what I know about it must be what the response option concludes.  The first inquiry is far simpler than the second.  And, if the first inquiry is not met, the response option cannot be correct.

Choice A?  First, do we know anything about Some D?  Well, some D are B (since All B are D).  So, the first (simple to determine) requirement is met.  At this point we can determine whether the second part must follow logically from the argument (i.e.; is this the correct response) or we can move forward and see if we can quickly eliminate the other response options before we do this work. 

Personally, even if I absolutely determine this response option is correct, I am going to look at the other response options anyway.  Therefore, I might as well look at them before I do the rest of the analysis about Choice A.

Choice B?  No.  This is quickly eliminated.  We know nothing about All D.

Choice C?  No.  This is quickly eliminated.  We know nothing about Some A who are Not B.

Choice D?  No.  Again, this is quickly eliminated.  We know nothing about Most A who are C.

Choice E?  No.  Again, we know nothing about Some Not (A and C).  How can we know if they are (B and Not D). 

So, we can be done if we want to be.  Choices B through E are definitely wrong.  But, in case I am not right, I like to go back and check Choice A.  However, since I have seen that Choices B through E are not correct, I can look at Choice A and ask myself why is this true rather than is this true.  That makes it easier to see.

In this case, we know Some D are B (the ones that are the B that are D).  But, do we know some D are C?  Well, we know some B are C (from the first sentence).  We know All B are D.  Therefore, some D must be C (the D that are also B).

[ssilver0210]

Pretend as if there is one group of 20 effective teachers. Picture the group in your mind if possible.

Within that group, all 20 will be good communicators.

Within that same group, most of the 20 will be eccentric.  You can assign a number here as well, though defining most is a bit difficult, but we can say 17 of the 20 teachers will be eccentric. The number isn't very important, as long as you realize that some of the effective teachers (all of whom are good communicators) are eccentric.

The 17 effective teachers who are eccentric must also be good communicators, because the 17 who are eccentric were among the original 20 who are good communicators.

So, because all the teachers are good communicators, and some of those same teachers are eccentric, it must be true based on the statements as we've been given that some good communicators are eccentric. In our particular case, 17 teachers who are good communicators must also be eccentric.

[Lindbergh]

Pretend as if there is one group of 20 effective teachers. Picture the group in your mind if possible.

Within that group, all 20 will be good communicators.

Within that same group, most of the 20 will be eccentric.  You can assign a number here as well, though defining most is a bit difficult, but we can say 17 of the 20 teachers will be eccentric. The number isn't very important, as long as you realize that some of the effective teachers (all of whom are good communicators) are eccentric.

The 17 effective teachers who are eccentric must also be good communicators, because the 17 who are eccentric were among the original 20 who are good communicators.

So, because all the teachers are good communicators, and some of those same teachers are eccentric, it must be true based on the statements as we've been given that some good communicators are eccentric. In our particular case, 17 teachers who are good communicators must also be eccentric.

Good description.  I would just choose the number 11 over 17, because it better captures the minimum requirements of "most."

[ssilver0210]

Very true, good point.