Last visit was: 23 Apr 2026, 20:59 It is currently 23 Apr 2026, 20:59
Home
Messages
Tests
Schools
Events
Close
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
Your Progress

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
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Go to My Error Log Learn more
Close
Request Expert Reply
Confirm Cancel
Back to Forum
Create Topic
User avatar
GianKR
User avatar
Joined: 23 Jun 2023
Last visit: 07 Jun 2024
Posts: 38
Own Kudos:
186
 [4]
Given Kudos: 102
Location: Morocco
Concentration: Marketing, Nonprofit
Schools: Said '27 Judge '26 Rotterdam'25 HBS '27 Yale '27
WE:Advertising (Non-Profit and Government)
Schools: Said '27 Judge '26 Rotterdam'25 HBS '27 Yale '27
Posts: 38
Kudos: 186
 [4]
A bibliophile has a big collection of both fiction and non-fiction 24 Apr 2024, 14:58
1
Kudos
Add Kudos
3
Bookmarks
Bookmark this Post
00:00
Start Timer
Pause Timer
Resume Timer
Show Answer
Hide
Show
History
E
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!

Difficulty:

45% (medium)

Question Stats:

73% (02:24) correct 27% (02:20) wrong based on 75 sessions

History

Date
Time
Result
Not Attempted Yet
­A bibliophile has a big collection of both fiction and non-fiction books in which non-fiction account for 40% of his books. On his birthday, all his friends gift him fiction books such that non-fiction now account for only 25% of his collection. By what percent has the number of fiction books increased in his collection? 

A) 30%
B) 50%
C) 60%
D) 80%
E) 100% ­
ShowHide Answer
Official Answer
E
User avatar
chetan2u
User avatar
GMAT Expert
Joined: 02 Aug 2009
Last visit: 22 Apr 2026
Posts: 11,229
Own Kudos:
45,005
Given Kudos: 335
Status:Math and DI Expert
Location: India
Concentration: Human Resources, General Management
GMAT Focus 1: 735 Q90 V89 DI81
Products:
My Reviews
Expert
Expert reply
GMAT Focus 1: 735 Q90 V89 DI81
Posts: 11,229
Kudos: 45,005
A bibliophile has a big collection of both fiction and non-fiction 24 Apr 2024, 18:58
Kudos
Add Kudos
Bookmarks
Bookmark this Post
GianKR
­A bibliophile has a big collection of both fiction and non-fiction books in which non-fiction account for 40% of his books. On his birthday, all his friends gift him fiction books such that non-fiction now account for only 25% of his collection. By what percent has the number of fiction books increased in his collection? 

A) 30%
B) 50%
C) 60%
D) 80%
E) 100% ­
Show more
­Let the number of books be x.

NUmber of non-fiction, n, = 40.

After the gifts were recieved, 40 = 0.25*t, where t is the new total. => t=160
Thus, 60 books were gifted, all of which were fiction.
Now, fiction books have gone up to 120 from 60. % = 100*60/60 = 100%

E
User avatar
GianKR
User avatar
Joined: 23 Jun 2023
Last visit: 07 Jun 2024
Posts: 38
Own Kudos:
186
Given Kudos: 102
Location: Morocco
Concentration: Marketing, Nonprofit
Schools: Said '27 Judge '26 Rotterdam'25 HBS '27 Yale '27
WE:Advertising (Non-Profit and Government)
Schools: Said '27 Judge '26 Rotterdam'25 HBS '27 Yale '27
Posts: 38
Kudos: 186
A bibliophile has a big collection of both fiction and non-fiction 25 Apr 2024, 16:02
Kudos
Add Kudos
Bookmarks
Bookmark this Post
­KarishmaB Can you please share with us the weighted averages method to solving this question that I came across on Anaprep? It has been intuitive to me to solve it using algebra but not so much using weighted averages. Thank you! 
User avatar
KarishmaB
User avatar
Joined: 16 Oct 2010
Last visit: 23 Apr 2026
Posts: 16,441
Own Kudos:
79,397
 [1]
Given Kudos: 484
Location: Pune, India
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 16,441
Kudos: 79,397
 [1]
A bibliophile has a big collection of both fiction and non-fiction 26 Apr 2024, 04:25
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
 
GianKR
­A bibliophile has a big collection of both fiction and non-fiction books in which non-fiction account for 40% of his books. On his birthday, all his friends gift him fiction books such that non-fiction now account for only 25% of his collection. By what percent has the number of fiction books increased in his collection? 

A) 30%
B) 50%
C) 60%
D) 80%
E) 100% ­
Show more
­
Think of it as mixtures. 
I am mixing two groups. One has 40% non fiction books. (His current collection). The other has 0% non fiction books (gifts). 
The overall mixture becomes 25% non fiction books. 

\(\frac{w1}{w2} = \frac{(A2 - Avg)}{(Avg - A1)} = \frac{(0 - 25)}{(25-40)} = \frac{5}{3}\)

So if he had 5 books total originally (3 fiction and 2 non fiction), his friends gifted him 3 fiction books. 
So number of fiction books have increased by 100% (3 has become 6).

Answer (E)

GianKR - It is just a matter of practice. We are so used to using algebra that we are unable to think beyond it. The learners who get used to weighted averages cannot see beyond weighted averages :)

Video links of weighted averages and mixtures:

https://www.youtube.com/watch?v=_GOAU7moZ2Q
https://www.youtube.com/watch?v=VdBl9Hw0HBg
 ­
User avatar
GianKR
User avatar
Joined: 23 Jun 2023
Last visit: 07 Jun 2024
Posts: 38
Own Kudos:
186
Given Kudos: 102
Location: Morocco
Concentration: Marketing, Nonprofit
Schools: Said '27 Judge '26 Rotterdam'25 HBS '27 Yale '27
WE:Advertising (Non-Profit and Government)
Schools: Said '27 Judge '26 Rotterdam'25 HBS '27 Yale '27
Posts: 38
Kudos: 186
A bibliophile has a big collection of both fiction and non-fiction 26 Apr 2024, 09:40
Kudos
Add Kudos
Bookmarks
Bookmark this Post
KarishmaB

GianKR
­A bibliophile has a big collection of both fiction and non-fiction books in which non-fiction account for 40% of his books. On his birthday, all his friends gift him fiction books such that non-fiction now account for only 25% of his collection. By what percent has the number of fiction books increased in his collection? 

A) 30%
B) 50%
C) 60%
D) 80%
E) 100% ­
­
Think of it as mixtures. 
I am mixing two groups. One has 40% non fiction books. (His current collection). The other has 0% non fiction books (gifts). 
The overall mixture becomes 25% non fiction books. 

\(\frac{w1}{w2} = \frac{(A2 - Avg)}{(Avg - A1)} = \frac{(0 - 25)}{(25-40)} = \frac{5}{3}\)

So if he had 5 books total originally (3 fiction and 2 non fiction), his friends gifted him 3 fiction books. 
So number of fiction books have increased by 100% (3 has become 6).

Answer (E)

GianKR - It is just a matter of practice. We are so used to using algebra that we are unable to think beyond it. The learners who get used to weighted averages cannot see beyond weighted averages :)

Video links of weighted averages and mixtures:

https://www.youtube.com/watch?v=_GOAU7moZ2Q
https://www.youtube.com/watch?v=VdBl9Hw0HBg
 ­
Show more
­If we would have instead mixed 60% fiction books (current collection) with 100% fiction books (gifts), obtaining an overall average of 75% fiction books, we would have still got the same results, correct? 

In my opinion the most difficult part about weighted averages is to actually recognise that it is a weighted average question. Do you have any quick tips with this regard? 

Thank you so much KarishmaB for your usual support! 
User avatar
KarishmaB
User avatar
Joined: 16 Oct 2010
Last visit: 23 Apr 2026
Posts: 16,441
Own Kudos:
79,397
 [1]
Given Kudos: 484
Location: Pune, India
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 16,441
Kudos: 79,397
 [1]
A bibliophile has a big collection of both fiction and non-fiction 26 Apr 2024, 09:49
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
GianKR

KarishmaB

GianKR
­A bibliophile has a big collection of both fiction and non-fiction books in which non-fiction account for 40% of his books. On his birthday, all his friends gift him fiction books such that non-fiction now account for only 25% of his collection. By what percent has the number of fiction books increased in his collection? 

A) 30%
B) 50%
C) 60%
D) 80%
E) 100% ­
­
Think of it as mixtures. 
I am mixing two groups. One has 40% non fiction books. (His current collection). The other has 0% non fiction books (gifts). 
The overall mixture becomes 25% non fiction books. 

\(\frac{w1}{w2} = \frac{(A2 - Avg)}{(Avg - A1)} = \frac{(0 - 25)}{(25-40)} = \frac{5}{3}\)

So if he had 5 books total originally (3 fiction and 2 non fiction), his friends gifted him 3 fiction books. 
So number of fiction books have increased by 100% (3 has become 6).

Answer (E)

GianKR - It is just a matter of practice. We are so used to using algebra that we are unable to think beyond it. The learners who get used to weighted averages cannot see beyond weighted averages :)

Video links of weighted averages and mixtures:

https://www.youtube.com/watch?v=_GOAU7moZ2Q
https://www.youtube.com/watch?v=VdBl9Hw0HBg
 ­
­If we would have instead mixed 60% fiction books (current collection) with 100% fiction books (gifts), obtaining an overall average of 75% fiction books, we would have still got the same results, correct? 

In my opinion the most difficult part about weighted averages is to actually recognise that it is a weighted average question. Do you have any quick tips with this regard? 

Thank you so much KarishmaB for your usual support! 
Show more
­You are right on both counts. 
Here is a video where I discuss how it could be used in different type of questions: 
https://youtu.be/6r8b09Qvd4E?feature=shared
Moderators:
Bunuel
Math Expert
109785 posts
Krunaal
Tuck School Moderator
853 posts