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Re: Combinatorics - Perms [#permalink]
17 Feb 2010, 02:21

bangalorian2000 wrote:

johnnymac wrote:

The answer is 13, not 35.

Question does not say that we cannot select duplicate letters. So to select 3 charaters from 7 characters hence 7C3.

This isn't correct. You need to take into account that some of the elements are duplicate. With your logic.. if u have the letters as {a,a,a,a,a,a,a}... then would no of comb possible for 3 letter word be 7c3? I don't think so.

Your answer would only be correct if all the 7 letters were different. If anyone of them repeated, the combinations would become less in number. Please check! _________________

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Re: How many combinations of three letters taken from letters [#permalink]
10 Aug 2012, 16:21

I always learn by modifying problems a little bit and see what answers I can come up with to test my understanding so...

If I assume that there's an extra a, an extra b, and an extra c for example. i.e. We have (a,a,a,b,b,b,c,c,c,d). Will that change our computation of 4C3+3C1*3C1? I myself don't think so but I'm waiting for your comments.

If I assume that's there's an an extra couple of D's and E's and 1 F i.e. (a,a,b,b,c,c,d,d,d,e,e,f). In this case the answer, as I guess, is 6C3+5C3*4C1=60. Am I correct?

Re: How many combinations of three letters taken from letters [#permalink]
23 Sep 2012, 07:22

Just got confused.

approaching by filling in slots.

we have three slots for seven letters.

now the first slot can be filled by any of the 7 letters, then 6, then 5.

so if there were 7 different letters, we would have had 7*6*5 and remove the repetitive combinations by dividing with 3! that makes 35 combinations.

For this approach, I got stuck here not knowing how to delete the repetitive combinations due to double letters. Help please.

P.S: I got the answer through other approach by adding unique and double letter combinations, I just want to understand why I got stuck above, rather how to continue from above.

Re: How many combinations of three letters taken from letters [#permalink]
25 Jan 2013, 11:36

Hey could someone please explain how they got 3C1 3C1? There are only two elements chosen in that situation? There doesn't seem to be a good explanation, or an OA for this problem _________________

Re: How many combinations of three letters taken from letters [#permalink]
26 Jan 2013, 04:09

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bmwhype2 wrote:

How many combinations of three letters taken from letters (a, a, b, b, c, c, d) are possible?

A. 12 B. 13 C. 35 D. 36 E. 56

This kind of question has little chances appearing on the actual test.

Anyway, we have 7 letters {a, a, b, b, c, c, d}. There are 2 ways to select 3 letters out of this set:

CASE #1: all letters are distinct:

Since there are 4 distinct letters a, b, c and d, then the # of ways to select 3 out of 4 is 4C3=4.

CASE #2: 2 letters are the same and the third is different:

There are 3 letters from the set which can provide us with two letters: a, b, and c. 3C1=3 gives the # of ways to select which letter out of these 3 will provide us with 2 letters. For, example double letters can be aa, bb, or cc.

Next, we are left with 3 letters to choose the third letter. For example, if we choose aa, then b, c, and d are left to choose from for the third letter, thus the # of ways to do that is 3C1=3.

Total # of ways for this case is therefore 3C1*3C1=9.

Re: How many combinations of three letters taken from letters [#permalink]
26 Jul 2014, 21:54

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Re: How many combinations of three letters taken from letters [#permalink]
24 Sep 2014, 06:57

bmwhype2 wrote:

How many combinations of three letters taken from letters (a, a, b, b, c, c, d) are possible?

A. 12 B. 13 C. 35 D. 36 E. 56

Sol:

consider any one pair with d one pair from 3 pairs = 3c1 *1 (AAD or BBD or CCD) But they can interchange their position(AAD,ADA,DAA) hence 3c1*1*3!/2! = 9

also we can consider all different digits

so we have 4 distinct objects and we have to choose 3 i.e. 4c3 = 4

Re: How many combinations of three letters taken from letters [#permalink]
06 Oct 2014, 04:44

johnnymac wrote:

jeeteshsingh - you use it to account for the following:

aa w/ b,c,d (aac, aab, aad) - 3 of these bb w/ a,c,d -3 of these cc w/ a,b,d -3 of these

3C1 is choose 3 letters w/ two of the same letter

It makes more sense to me as 3*3C1 = 9 total (rather than 3C1*3C1)

Indeed writing it as : 2C2*3C1*3=9 will make the most sense and clear it to almost anybody. ( 2C2 refers to the choosing of same 2 letters and then 3C1 refers to chossing any one letter from among the 3 diff types of letters left and then finally *3 gives the total such possible discrete cases.)

Hope this helps!! _________________

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Re: How many combinations of three letters taken from letters
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06 Oct 2014, 04:44

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