Thanks for the follow up. Let me explain:
As per the passage, the excerpts can reach anyone - so people who would have would otherwise as well as people who would not have bought otherwise are included. So, let's understand the impact on each category.
Category 1- People who would have bought the books if excerpts were not published (100 in previous example):
- Some of them will not be affected, they will still buy the book - No impact.
- Some of them will not purchase the book (8 in the previous example) - Reduces sale
- So, no additional sales because of excerpts in this category- net effect is reduction in sales.
Category 2- People who would not have bought the books if excerpts were not published (let's say 50- these are completely different from 100 above):
- Some of them will not be affected, they will still not buy the book - No impact.
- Some of them will be motivated to purchase the book (2 in the previous example) - Increases sale
- Net effect is increase in sales.
Overall impact = Net increase - Net reduction
Net sales will increase only if #people who got motivated to buy the book is more than the #people who get demotivated to buy the book.
There is a slight jump here (an assumption that all people have been reached - which get covered by "high circulation" to an extent, but not completely). Usually, you will not find such jumps.
Question Type - The correct answer to such questions will be a correct Inference.
kabirgandhi
Hello, thank you for the reply

To get to the reasoning that you have shared, would we have to assume that the except is reaching only those who would have bought the book anyway?
If these 10 people would not have necessarily bought the book, then even if A) were true, we would get 2 additional people who would've bought the book, over and above.
Adding on to this - I wanted to ask you a question about these types of problems.
When the stem says: Which of the following conclusions is best supported by the passage?
Is that the same as: which of the following must be true, given the information above?
The latter is clearly and inference question, whereas I have always struggled to identify whether the former should be treated the same way - do we have to look for something that absolutely MUST be true? Or just a logical conclusion following from the passage?