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# There are 4 books on physics and 2 books on maths.Find the

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Intern
Joined: 29 Aug 2003
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There are 4 books on physics and 2 books on maths.Find the [#permalink]  11 Dec 2003, 03:39
There are 4 books on physics and 2 books on maths.Find the no. of ways in which these books can be arranged so that the books on maths are not together!
Director
Joined: 13 Nov 2003
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6!-4!x3!=720-144=576...
Senior Manager
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Intern
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No..this is not the answer!!! Its 480.
I m wondering how to get 480???
Director
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total number we can order the 6 books is 6!. When we take 4 maths together as one and the other 2 we can order these in 3! ways but for each of these orderings we will have 4! orderings of the 4 math books. Total number when maths are not together is 6!-4!x3!
Director
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Sorry but I got it other way round, i calculated for 4 mats and 2 physics. . when it is 4 physics and 2 maths it should be 6!-5!x2=720-240=480
Senior Manager
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there are 4 PHYSICS books and not math books.

using BG's CORRECT LOGIC ...

2 math books as 1 + 4 other books = 5!
2 math books can be arranged in 2! = 2 ways
# of ways which have 2 math books together = 5! * 2 = 120 * 2 = 240
# of total ways = 6! = 720

# of ways which DON'T have the 2 math books together = 720 - 240 = 480
Senior Manager
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well was typing when BG already posted his correction! so you can ignore my post
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Re: Probability!!! [#permalink]  12 Dec 2003, 23:20
4128851 wrote:
There are 4 books on physics and 2 books on maths.Find the no. of ways in which these books can be arranged so that the books on maths are not together!

total # of ways of arranging these "distinguishable" books

6! = 720

ways in which math books are ALWAYS together.

5! *2! =240

required # of ways = 720 - 240 = 480
Re: Probability!!!   [#permalink] 12 Dec 2003, 23:20
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