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How many different ways to pick 3 letters from ABCDEFG, when

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boksana
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How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   27 Nov 2004, 17:59
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How many different ways to pick 3 letters from ABCDEFG, when AB can't be together?



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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   27 Nov 2004, 21:57
To choose 3 letters out of ABCDEFG without AB together :
we have to consider AB as ONE letter

so we have to choose 3 letters out of 6(since AB are one letter)

No. of ways to choosing are c(6,3) = 20.

What is the answer ?
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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   Updated on: 28 Nov 2004, 18:25
There are 3 ways in which AB cant be together.

A Not B or B not A or Neither A Nor B
A Not B = 3(1. 5.4) = 60
B Not A = 3(1.5.4) = 60
Neither A Nor B = 5.4.3 = 60

180, if order matters.

If order doesnt matter: 5c2+5c2+5c3 = 30.

Originally posted by prep_gmat on 28 Nov 2004, 10:47.
Last edited by prep_gmat on 28 Nov 2004, 18:25, edited 1 time in total.
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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   28 Nov 2004, 15:06
Various ways : 7C3

AB together : 5!

So 7C3-5!
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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   28 Nov 2004, 18:27
Twixt,
7c3 - 5! would yield a negative number?.
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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   28 Nov 2004, 20:16
oops late on this one..

2C1*5C2 + 5C3 = 30.

2C1 because - either of A or B.
5C2 2 of the remaining 5 letters

5C3 - pick all three letters from the 5.
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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   29 Nov 2004, 03:57
I am sorry my answer was 7C3 - 5 and not 7C3 - 5! (5! makes no sense since there is only one slot left with 5 letters to put into...)

=5.6.7/6-5=35-5=30
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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   29 Nov 2004, 05:45
In a question like this do we assume that the order of the letters do not matter? :?
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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   29 Nov 2004, 11:15
To Twist & Vprabhala,

Your answer is 7C3-5. How do you figure out that "5"?
Write out all the possibilities like the attached file?
Is there any better way to find this number?

:stupid
Attachments

letters.doc [19 KiB]
Downloaded 82 times

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twixt
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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   29 Nov 2004, 13:38
As order does not matter, you have to consider a 3 slots box. A is in 1st slot, B in the 2nd one, then you can put any of the 5 remaining letters in the last one.
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gayathri
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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   29 Nov 2004, 16:38
twixt wrote:
As order does not matter, you have to consider a 3 slots box. A is in 1st slot, B in the 2nd one, then you can put any of the 5 remaining letters in the last one.


Why is it assumed that order does not matter? :stupid
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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink] How many different ways to pick 3 letters from ABCDEFG, when   30 Nov 2004, 10:31
as it is not mentioned.. we choose the lesser evil :roll:



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Re: How many different ways to pick 3 letters from ABCDEFG, when [#permalink]
30 Nov 2004, 10:31
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