Total books and average books

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Total books and average books

by jagan.y » Mon Jun 24, 2013 9:56 pm
30 years ago residents of Pandora County used to buy an average of 10 books every year. Today the residents of Pandora County buy an average of 3 books every year. Therefore it can be concluded that book sales in Pandora County must have fallen over these 30 years.
The argument rests in which of the following assumptions?
A. The residents of Pandora County used to have more free time 30 years ago than they do no
B. The residents of Pandora county have many more activities that vie for their attraction today than 30 years
C. The population of Pandora County has not increased significantly in the last 30 years
D. The literacy rate hasn't significantly changed in Pandora County over the past 30 years
E. Those residents of Pandora County who used to read 30 years ago have now not become too old to read
Source: Aristotle CR Grail
[spoiler]OA: C[/spoiler]

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by ceilidh.erickson » Tue Jun 25, 2013 9:15 am
When you're faced with a "find the assumption" question on CR, your job is to identify the logical gap in the reasoning of the argument. One way to do that is to ask yourself - is it possible for the premises to be true, but for the conclusion not to be?

We're given that per-person book consumption has fallen from 10 books per year to 3 books per year. Is there any way that overall book sales would not have fallen? Sure - if there were more people in the county overall. Clearly the author is assuming that the population remained constant. If that assumption were true, then a per-capita sales decline would mean an overall decline.

C is the only answer choice that speaks to that assumption. All of the others deal with explanations for why this decline might have happened, but we don't know - or care - why. We just need to resolve the missing link between per-capita (proportional) information and total number information.

Pay particular attention to gaps between PROPORTIONAL information and REAL-NUMBER information, or for any time the metric used to measure something changes. It's a popular logical gap type on the GMAT (both on CR and DS). For more information, see these posts:

https://www.beatthegmat.com/i-m-doubting ... tml#551227
https://www.beatthegmat.com/cr-evaluate- ... tml#558393
https://www.beatthegmat.com/statistics-c ... tml#564609
Ceilidh Erickson
EdM in Mind, Brain, and Education