Math Is Fun Forum
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#1 2016-07-26 21:45:01
- Srikantan
- Member
- From: Tanjore
- Registered: 2016-06-04
- Posts: 42
Prob
A man has written four letters to his friends whose addresses are correctly written on four envelopes. His driver puts the four letters arbitrarily in the four envelopes , pastes and posts them. In how many ways will no letter go to the right person?
Last edited by Srikantan (2016-07-27 01:15:59)
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#2 2016-07-26 22:24:12
- thickhead
- Member
- Registered: 2016-04-16
- Posts: 1,086
Re: Prob
{1}Vasudhaiva Kutumakam.{The whole Universe is a family.}
(2)Yatra naaryasthu poojyanthe Ramanthe tatra Devataha
{Gods rejoice at those places where ladies are respected.}
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#3 2016-07-26 23:48:36
- bobbym
- bumpkin
- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606
Re: Prob
Hi;
In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.
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#4 2016-07-27 00:32:22
- Srikantan
- Member
- From: Tanjore
- Registered: 2016-06-04
- Posts: 42
Re: Prob
How is that derived?
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#5 2016-07-27 00:48:17
- bobbym
- bumpkin
- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606
Re: Prob
I have done these a couple of times in here alone. There is a formula for it.
In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.
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#6 2016-07-27 04:41:01
- thickhead
- Member
- Registered: 2016-04-16
- Posts: 1,086
Re: Prob
Yes bobbym, Now I remember. It is (n-1)! when the interchange goes through the complete group in a loop. Derangement allows for smaller loops also. Therefore the number is larger.
{1}Vasudhaiva Kutumakam.{The whole Universe is a family.}
(2)Yatra naaryasthu poojyanthe Ramanthe tatra Devataha
{Gods rejoice at those places where ladies are respected.}
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#7 2016-07-27 04:50:59
- thickhead
- Member
- Registered: 2016-04-16
- Posts: 1,086
Re: Prob
Hi Srikantan,
This is bad. After getting the solution you should not withdraw a part of the problem. Please let others also see it.
{1}Vasudhaiva Kutumakam.{The whole Universe is a family.}
(2)Yatra naaryasthu poojyanthe Ramanthe tatra Devataha
{Gods rejoice at those places where ladies are respected.}
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#8 2016-07-27 05:20:43
- Bob
- Administrator
- Registered: 2010-06-20
- Posts: 10,143
Re: Prob
I'm not saying that is what happened here but sometimes new posters put up their homework and then panic because they worry their teacher will see they haven't done the question themselves. A few years ago someone posted a question that was part of a competition. The rules stated that competitors should not seek help on the internet. The question master did a search and uncovered the post. Hhhm.
When someone asks for help with a problem I try to give the minimum that will enable the poster to fill in the gaps themselves. That way they get maximum benefit from the 'help'. Of course, it is tricky knowing what 'minimum' means for any particular poster which is why the 'sticky' at the head of the help me section says (amongst other things) "Tell us what level you are at. It will help avoid replies that are too easy or too hard for you to understand." and "make it clear what kind of help you want."
Bob
Children are not defined by school ...........The Fonz
You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you! …………….Bob 
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