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.

It appears that you are browsing the GMAT Club forum unregistered!

Signing up is free, quick, and confidential.
Join other 500,000 members and get the full benefits of GMAT Club

Registration gives you:

Tests

Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.

Applicant Stats

View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more

Books/Downloads

Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

1) each French book has to be separated at least by one English book, so we have following order: *FE*FE*FE*FE*FE*FE*FE*FE*F* where * denotes 10 possible places for remained 11-8=3 English books

2) Now, let's count in how many ways we can place remained English books:

\(C^{10}_3\) - all 3 books at distinct places. \(P^{10}_2\) - 2 books together and remained book at distinct place (order is important). \(C^{10}_1\) - all 3 books together.

maybe there is a better solution _________________

Bunuel, I wish I could adopt your way to look at the problem. Sometimes, it's an easy one lost in translation. Walker, though your approach is different, but it's pretty interesting. Thanks again guys, appreciate your help here.

I am not very clear guys. Buenel, in your solution you have only considered the different ways that the 9 french books can be placed in 12 position. However, even the English books can be placed in their respective slots in more than one way. i.e. let the English books be 1e, 2e, 3e etc. Now different placement of these english books can yield more possible combinations.

F 1e F 2e F 3e or F 3e F 1e F 2e or F 2e F 1e F 3e

It is true that English books may have different positions but that's not what the question is asking. It's only a matter of placing 9 books in 12 slots.

I am not very clear guys. Buenel, in your solution you have only considered the different ways that the 9 french books can be placed in 12 position. However, even the English books can be placed in their respective slots in more than one way. i.e. let the English books be 1e, 2e, 3e etc. Now different placement of these english books can yield more possible combinations.

F 1e F 2e F 3e or F 3e F 1e F 2e or F 2e F 1e F 3e

Hope I am clear in expressing my doubt

Think I understand your point. You are saying that along with arrangements with no French books being adjacent, English and French books themselves could be arranged in different ways. But I don't think that this is the case. Though it's quite ambiguous question in a sense. Basically when GMAT wants us to consider some items as distinct it specifies this OR it's quite obvious. In original question we don't know whether these books are distinct or not: maybe all French and English books are the same, maybe not, we don't know that.

If the question were: how can we arrange 11 boys and 9 girls so that no girls are together, then the answer would be 12C9*11!*9!. As it's obvious that they are all different.

If the question were: how can we arrange 11 A-s and 9 B-s so that no B-s are together, then the answer would be 12C9. As it's obvious that A-s and B-s are the same.

11 english books can be arranged in 11 slots (say) in 11! ways. There will be 10 empty spaces between adjacent english books and another 2 empty spaces at either ends into which the french books can be placed such that no two french books are adjacent to one another. This is done in 12C9 ways.

Total number of ways should therefore be 12C9 * 11!. _________________

11 english books can be arranged in 11 slots (say) in 11! ways. There will be 10 empty spaces between adjacent english books and another 2 empty spaces at either ends into which the french books can be placed such that no two french books are adjacent to one another. This is done in 12C9 ways.

Total number of ways should therefore be 12C9 * 11!.

Not so.

Let's say we have A1, A2, B1, B2 (meaning that A-s and B-s are distinct). We want to arrange them so that no B-s are adjacent:

*A1*A2* and we can place B1 and B2 in 3 empty slots. It can be done in 3C2 # of ways. BUT A1 and A2 can be arranged like *A1*A2* OR *A2*A1*, plus B1 and B2 can be arranged as B1B2 or B2B1.

Total # of ways 3C2*2!*2!=12.

Still if not convinced: B1,A1,B2,A2 B1,A2,B2,A1 B2,A1,B1,A2 B2,A2,B1,A1

A1,B1,A2,B2 A2,B1,A1,B2 A1,B2,A2,B1 A2,B2,A1,B1,

B1,A1,A2,B2 B1,A2,A1,B2 B2,A1,A2,B1 B2,A2,A1,B1

BUT again this is the case when we have DISTINCT items. So, if we were told that all French book are different and all English books are different, then the answer would be 12C9*11!*9!.

In our original question we are not told that French books are different and are not told that English books are different. So # of ways would be 12C9.

Let's consider the easier example: # of ways to arrange two A-s and 2 B-s so that no B-s are adjacent: 3C2=3.

Thanks Buenel. Ya, It helps, here I was confused because it does not mention whether we should consider the English Books and the French books to be distinct, or as you said, like any A's and B's, which reduces the total number of possible arrangements.

Re: In how many ways can 11 books on English and 9 books on [#permalink]

Show Tags

17 Aug 2014, 23:35

Hi Bunuel !!

In the question it is mentioned that - ' 11 books on English and 9 books on French be placed in a row '.

As per the question since it says 11 books on English and 9 books on , it is implicitly saying that these books of French and English are of different subjects, therfore are distinct. I doubt if we should consider these books as similar.

So the answer should be: 12P9.11!

Please correct me if there is a gap in my understanding.

gmatclubot

Re: In how many ways can 11 books on English and 9 books on
[#permalink]
17 Aug 2014, 23:35

Part 2 of the GMAT: How I tackled the GMAT and improved a disappointing score Apologies for the month gap. I went on vacation and had to finish up a...

I’m a little delirious because I’m a little sleep deprived. But whatever. I have to write this post because... I’M IN! Funnily enough, I actually missed the acceptance phone...

So the last couple of weeks have seen a flurry of discussion in our MBA class Whatsapp group around Brexit, the referendum and currency exchange. Most of us believed...

This highly influential bestseller was first published over 25 years ago. I had wanted to read this book for a long time and I finally got around to it...