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# In how many ways can the letters of the word "COMPUTER"

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Manager
Joined: 16 Apr 2010
Posts: 206
In how many ways can the letters of the word "COMPUTER" [#permalink]

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27 Jul 2010, 20:50
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Hello,

What is the shortest and most systematic way of solving the below question?

In how many ways can the letters of the word "COMPUTER" be arranged if vowels occupy the even positions?

regards,
Jack
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Joined: 02 Sep 2009
Posts: 47037

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28 Jul 2010, 00:12
jakolik wrote:
Hello,

What is the shortest and most systematic way of solving the below question?

In how many ways can the letters of the word "COMPUTER" be arranged if vowels occupy the even positions?

regards,
Jack

I think it's meant that vowels can occupy only even positions:
C|O|M|P|U|T|E|R
O|E|O|E|O|E|O|E (O and E stand for odd and even positions respectively).

# of arrangements would be $$C^3_4*3!*5!=4!*5!=2880$$.

$$C^3_4$$ - choosing which 3 even positions out of 4 will be occupied by vowels (there are 4 even positions: 2nd, 4th, 6th and 8th and only 3 vowels);
$$3!$$ - # of different arrangements of these vowels on their even positions;
$$5!$$ - # of different arrangements of 8-3=5 other letters left.

Hope it helps.
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Joined: 03 Mar 2010
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28 Jul 2010, 00:18
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1
The word COMPUTER consists of 8 letters of which 3 vowels. All letters are distinc.
First, put 3 vowels into 3 of the positions 2, 4, 6, and 8. There are 4*3*2=24 ways.
Second, once the positions of the 3 vowels are determined, we have to put the 5 other letters into the remaining positions. There are 5*4*3*2*1=120 ways.
Therefore, there are a total of 120*24=2880 ways.
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Re: In how many ways can the letters of the word "COMPUTER" [#permalink]

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23 Sep 2017, 22:25
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Re: In how many ways can the letters of the word "COMPUTER"   [#permalink] 23 Sep 2017, 22:25
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