Hi Diophantus,

Are you actually studying for the GMAT? I ask because you've posted a number of questions that are not written in proper GMAT "style" and without answer choices. On that second point, we're essentially stuck doing math (instead of using Tactics and potentially taking advantage of the built-in patterns that appear all over the GMAT). For example, this question would clearly be solvable by TESTing THE ANSWERS - the correct answer would 'fit' the given information perfectly. While you could set up a 2-variable 'system' of equations to answer this question, you can also use 'brute force' math to get to the solution:
Assuming each notebook costs the same, with $200, you could have the following situations:
$10/book gets you 20 books
$20/book gets you 10 books
$40/book gets you 5 books
$50/book gets you 4 books
$100/book gets you 2 books

Looking at this list, there's only one situation in which raising the price of a notebook $10 results in 1 fewer book purchased: When the original price is $40/book.

GMAT assassins aren't born, they're made,
Rich