Add-maths question

[abcde]

Regarding permutations, there is this question in the book which i could not solve despite many tries.
Q)24. 9 different books are to be arranged on a book shelf. 4 of these books are written by Shakespeare, 2 by Dickens and 3 by Conrad.How many possible permutations are there if
(c) the books by Conrad are seperated from each other?
I keep on getting the answer: 9!- (3!x 7!)=332,640 whereas the ans at the book's back is 151,200.
Kindly help. anyone?

 

[Pals_1010]

You have only considered when the 3 Conrad books are together.

You should also subtract from 9! the case where 2 Conrad books are together.

 

[abcde]

You mean i should do this: 9!- [ (3!x7!)+(2!x8!)] ? that still gives the wrong answer of 252,000. Any more suggestions? Where am i wrong? What is the correct answer. Please reply asap!

 

[OakMoon!]

It took me alot of time to find the solution to this question back in may and I still remember it. Even though I did take out the answer given at the back it didn't make any sense. It came out something like this: 7P3*6P6=151200 or 7i/4i * 6i
You can make the sense out of it yourself. There was another way I solved this question by subtraciting. But I have to solve this question again and it will take time. So you need to wait.

 

[OakMoon!]

yar you subtracted when 3 books are together and then you subtracted when two books are together. but you didnt find out the 2 books together permutation correctly. because when the two books are together there is a chance the third book will come and sit beside them it self according to the permutation used. So revise the same thing you have done. But you need to subtract one other thing. Out of the 7 books left apart from the two books there is one comrad too. And there is a chance that the one comrad book is placed beside the 2 books and so they ll be together again. you have to subtract that too.

 

[OakMoon!]

here it is. 9!-8!*3!+7!*3!.. = 151200

 

[abcde]

I still don't get it! I understand your point but the working is not clear to me. Kindly elaborate that. Moreover, how should i practice for my add-maths exam? Its in november.

 

[OakMoon!]

Just practice past papers. Make sure you do past papers of the last 10 years. Do every paper. And then check your paper using marking schemes and mark yourself.
Btw I can't elaborate the answer. As I told you I am also not clear about it. Never was. :S

 

[abcde]

Yes, the question is confusing. If anyone has any more suggestions, please reply. Thank you hamidali391 !

 

[Snowysangel]

here it is. 9!-8!*3!+7!*3!.. = 151200

Do you remember how you got that?

 

[OakMoon!]

Do you remember how you got that?

Read the posts above.