It is currently 20 Mar 2018, 01:14

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# M09-13

Author Message
TAGS:

### Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 44332

### Show Tags

16 Sep 2014, 00:39
Expert's post
5
This post was
BOOKMARKED
00:00

Difficulty:

35% (medium)

Question Stats:

71% (01:21) correct 29% (01:26) wrong based on 102 sessions

### HideShow timer Statistics

Each shelf of a bookcase contained 12 books. If the librarian took out 21 books and rearranged the remaining books so that all shelves but the last one contained 8 books and that last shelf contained 11 books, how many shelves does the bookcase have?

A. 5
B. 6
C. 7
D. 8
E. 9
[Reveal] Spoiler: OA

_________________
Math Expert
Joined: 02 Sep 2009
Posts: 44332

### Show Tags

16 Sep 2014, 00:39
Expert's post
1
This post was
BOOKMARKED
Official Solution:

Each shelf of a bookcase contained 12 books. If the librarian took out 21 books and rearranged the remaining books so that all shelves but the last one contained 8 books and that last shelf contained 11 books, how many shelves does the bookcase have?

A. 5
B. 6
C. 7
D. 8
E. 9

Say there are total of $$x$$ shelves, then: $$12*x=21+8*(x-1)+11$$, which gives $$x=6$$.

_________________
Intern
Joined: 21 May 2013
Posts: 8

### Show Tags

24 Oct 2014, 23:50
Bunuel wrote:
Official Solution:

Each shelf of a bookcase contained 12 books. If the librarian took out 21 books and rearranged the remaining books so that all shelves but the last one contained 8 books and that last shelf contained 11 books, how many shelves does the bookcase have?

A. 5
B. 6
C. 7
D. 8
E. 9

Say there are total of $$x$$ shelves, then: $$12*x=21+8*(x-1)+11$$, which gives $$x=6$$.

Hi Bunuel,

Thanks for the explanation. However, why do you subtract "1" from 8*(x-1)? Can you please verify?

Thank you so much.
Aram
Math Expert
Joined: 02 Sep 2009
Posts: 44332

### Show Tags

25 Oct 2014, 06:41
safaria25 wrote:
Bunuel wrote:
Official Solution:

Each shelf of a bookcase contained 12 books. If the librarian took out 21 books and rearranged the remaining books so that all shelves but the last one contained 8 books and that last shelf contained 11 books, how many shelves does the bookcase have?

A. 5
B. 6
C. 7
D. 8
E. 9

Say there are total of $$x$$ shelves, then: $$12*x=21+8*(x-1)+11$$, which gives $$x=6$$.

Hi Bunuel,

Thanks for the explanation. However, why do you subtract "1" from 8*(x-1)? Can you please verify?

Thank you so much.
Aram

We are told that all shelves but the last one contained 8 books, so the number of shelves with 8 books is x - 1.
_________________
Current Student
Joined: 10 Aug 2014
Posts: 50
GMAT 1: 680 Q49 V34

### Show Tags

10 Sep 2015, 19:35
What if we plug in values?
For example we have 6 shelves, thus 6*12=72 books - remove 21 books=51-11 books=40/8=5 shelves left+1 with 11 books=6 justify
and 8 shelves, 8*12=96-21=75-11=64/8=8 shelves with 8 books + 1 shelf with 11 books=9 thus 8 doesn't equal 9. So we have only 6 is the correct choice.
Intern
Joined: 10 May 2017
Posts: 27

### Show Tags

25 Jun 2017, 02:57
I used this method.

Total books current = 12 x n (where n is no of shelves)

21 books removed and re-arranged,

12n-21 = 8n + 3 (Since the last shelf has additional 3 books)

n=6.
Intern
Joined: 25 Aug 2017
Posts: 34

### Show Tags

30 Oct 2017, 17:52
Could someone please help explain to me why the following equation would not work for this problem? I got 8, which is the wrong answer, but am trying to figure out where my thought process crapped out on me. Thanks for your help.

12 books per shelf minus 21 books = 8 books per shelf + 11 on the last shelf
12x - 21 = 8x + 11
x = 8

Are the 21 books removed from equation all together?
Math Expert
Joined: 02 Sep 2009
Posts: 44332

### Show Tags

30 Oct 2017, 21:00
1
KUDOS
Expert's post
Raffio wrote:
Could someone please help explain to me why the following equation would not work for this problem? I got 8, which is the wrong answer, but am trying to figure out where my thought process crapped out on me. Thanks for your help.

12 books per shelf minus 21 books = 8 books per shelf + 11 on the last shelf
12x - 21 = 8x + 11
x = 8

Are the 21 books removed from equation all together?

_________________
SVP
Joined: 26 Mar 2013
Posts: 1506

### Show Tags

31 Oct 2017, 14:38
Raffio wrote:
Could someone please help explain to me why the following equation would not work for this problem? I got 8, which is the wrong answer, but am trying to figure out where my thought process crapped out on me. Thanks for your help.

12 books per shelf minus 21 books = 8 books per shelf + 11 on the last shelf
12x - 21 = 8x + 11
x = 8

Are the 21 books removed from equation all together?

The problem in highlighted part. You missed that All but one. So it should be 8(x -1). What you did is that you multiplied by 8x and add another self with 11.
Re: M09-13   [#permalink] 31 Oct 2017, 14:38
Display posts from previous: Sort by

# M09-13

Moderators: chetan2u, Bunuel

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.