PT20.S4.Q19 - nearly all mail that is correctly addressed

DMo-UTexas

http://7sage.com/lsat_explanations/lsat-20-section-4-question-19/

This question is giving me a hard time. I am pretty sure I have translated correctly. Please chime in if you know where I am going wrong.

It is a MBT ?
First Premise: MCA (Mail that is correctly addressed) --most--> A2D (Arrives in 2 Days)
Second: MCA --> [/A2D] --> DT (Damaged in Transit) - The sentence says "MCA [takes longer than this] only when DT." I think this is where my mistake is, but we will see.
Third: /A2D - most mail arrives in 3 or more days

The Answer: A large proportion of mail is incorrectly addressed

sicsemper

First of all, hook 'em.

Second, it's definitely a MBT. Your formulation of the second premise is a bit confusing. I would read that to say that "if a mail piece is correctly addressed, THEN it must not arrive in 2 days, THEN it must be damaged in transit". (Think of what the arrow means in your diagram.) I might change that to drop the first arrow such that it says MCA AND greater than two days -> damaged.

I'm also not sure what your third premise means.

I actually think though that the second premise is not necessary to your getting the right answer. The first premise says that NEARLY ALL correctly addressed mail arrives within two days (so definitely >50%). They've set up a binary so there is going to be mail that is correctly addressed and mail that is not correctly addressed (there are at this level only two types of mail). So if more than 50% of the mail arrives in three days or more, and nearly all of one type of mail (correctly addressed mail) arrives in fewer than two days, then we KNOW that that there must be "a lot of" incorrectly addressed mail - a nonzero portion of which must arrive in three days or more - in order to satisfy the first claim that more than 50% of the total mail population arrives in three days or later.

It's a pretty weak MBT - as are a lot of them. But I think the best way to arrive at this is to quickly go through all the options:
A - Irrelevant, as it still must be the case that "nearly all" mail in the correctly addressed category arrives in two days or fewer.
B - Way too strong. Nothing in the text to support this. Only necessary that some portion of the incorrectly addressed mail arrives later than two days, not that no portion arrives in two days or fewer.
C - Does not need to be the case. Consider the weighting of the two types of mail (correctly addressed and not correctly addressed). If not correctly addressed makes up 90% (or some high percentage) of the total pool, it could be the case that incorrectly addressed mail makes up most of both letters arriving in two days or fewer and three days or more.
D - Correct.
E - Dismiss as the the division thus far has been between "less than or equal to two" and "greater than or equal to three". (Nowhere else have we considered portions of days.)

J.Y. Ping

@sicsemper, that's a really good explanation!

@DMo-UTexas:
http://7sage.com/lsat_explanations/lsat-20-section-4-question-19/

DMo-UTexas

Yes, SicSemper, amazing response. I appreciate that you have noticed my alma mater, as well. Thank you guys.