Hi ziyuenlau,
This is a poorly-worded question, but I think that the 'intent' of the question is this:
"There are 1280 books in a library. Each person who borrowed books from this library borrowed from 1 to 10 (inclusive) books from the library. If all the books were borrowed just once each, then is the number of people who borrowed books greater than 240?"
This is a YES/NO question and the 'key' to solving it is to consider whether it's possible to have MORE than 240 people or not. We can TEST VALUES to prove the possibilities...
1) 65 people borrowed either 1 or 2 books
IF....
65 people borrowed just 1 book each, then there are 1215 books remaining.
Those 1215 books COULD have been borrowed by 400 people (borrowing 3-4 books each), so the total number of people would have been 65 + 400 = 465.... and the answer to the question would be YES.
Those 1215 books COULD have been borrowed by 122 people (borrowing 9-10 books each), so the total number of people would have been 65 + 122 = 187.... and the answer to the question would be NO.
Fact 1 is INSUFFICIENT
2) 120 people borrowed either 3 or 4 books
IF....
120 people borrowed 3 books each, then there are 920 books remaining.
Those 920 books COULD have been borrowed by 920 people (borrowing 1 book each), so the total number of people would have been 120 + 920 = 1040.... and the answer to the question would be YES.
Those 920 books COULD have been borrowed by 92 people (borrowing 10 books each), so the total number of people would have been 120 + 92 = 212.... and the answer to the question would be NO.
Fact 2 is INSUFFICIENT
Combined, we know....
65 people borrowed either 1 or 2 books
120 people borrowed either 3 or 4 books
After we consider the above 185 people, everyone else would have borrowed 5-10 books each. That's important because there will be at least a certain minimal number of extra people that there could have been....and we can determine that limit by considering the minimum and maximum numbers that we could start off with:
MINIMUM:
(65 people)(1 book each) = 65 books
(120 people)(3 books each) = 360 books
So those 185 people borrowed 425 books - at the minimum - before we consider anyone else. That leaves 855 books for the other people. This means AT LEAST 86 additional people borrowed books - since (85)(10) = 850 and that isn't enough books. 65+120+86 = 271 people and the answer to the question is YES.
MAXIMUM:
(65 people)(2 books each) = 130 books
(120 people)(4 books each) = 480 books
So those 185 people borrowed 610 books - at the maximum - before we consider anyone else. That leaves 670 books for the other people. This means AT LEAST 67 additional people borrowed books - since (67)(10) = 670. 65+120+67 = 252 people and the answer to the question is YES.
Thus, in all possibilities, there would be MORE than 240 people, so the answer to the question is ALWAYS YES.
Final Answer: C
GMAT assassins aren't born, they're made,
Rich
