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A researcher plans to identify each participant in a certain [#permalink]
17 Jun 2012, 03:13

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Difficulty:

45% (medium)

Question Stats:

55% (02:04) correct
45% (01:22) wrong based on 440 sessions

A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetical order. What is the least number of letters that can be used if there are 12 participants, and each participant is to receive a different code?

Re: A researcher plans to identify each participant in a certain [#permalink]
17 Jun 2012, 03:24

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sarb wrote:

A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetical order. What is the least number of letters that can be used if there are 12 participants, and each participant is to receive a different code?

A. 4 B. 5 C. 6 D. 7 E. 8

Say there are minimum of n letters needed, then;

The # of single letter codes possible would be n itself; The # of pair of distinct letters codes possible would be C^2_n (in alphabetical order);

Re: A researcher plans to identify each participant in a certain [#permalink]
17 Jun 2012, 03:34

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sarb wrote:

A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetical order. What is the least number of letters that can be used if there are 12 participants, and each participant is to receive a different code?

Re: A researcher plans to identify each participant in a certain [#permalink]
17 Jun 2012, 03:49

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Almost identical question:

John has 12 clients and he wants to use color coding to identify each client. If either a single color or a pair of two different colors can represent a client code, what is the minimum number of colors needed for the coding? Assume that changing the color order within a pair does not produce different codes. A. 24 B. 12 C. 7 D. 6 E. 5

The concept is not that hard. We can use combination or trial and error approach.

Combination approach: Let # of colors needed be n, then it must be true that n+C^2_n\geq{12} (C^2_n - # of ways to choose the pair of different colors from n colors when order doesn't matter) --> n+\frac{n(n-1)}{2}\geq{12} --> 2n+n(n-1)\geq{24} --> n(n+1)\geq{24} --> as n is an integer (it represents # of colors) n\geq{5} --> n_{min}=5.

Trial and error approach: If the minimum number of colors needed is 4 then there are 4 single color codes possible PLUS C^2_4=6 two-color codes --> 4+6=10<12 --> not enough for 12 codes;

If the minimum number of colors needed is 5 then there are 5 single color codes possible PLUS C^2_5=10 two-color codes --> 5+10=15>12 --> more than enough for 12 codes.

Actually as the least answer choice is 5 then if you tried it first you'd get the correct answer right away.

Re: A researcher plans to identify each participant in a certain [#permalink]
23 Dec 2012, 23:41

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Expert's post

eaakbari wrote:

Bunuel wrote:

sarb wrote:

A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetical order. What is the least number of letters that can be used if there are 12 participants, and each participant is to receive a different code?

A. 4 B. 5 C. 6 D. 7 E. 8

Say there are minimum of n letters needed, then;

The # of single letter codes possible would be n itself; The # of pair of distinct letters codes possible would be C^2_n (in alphabetical order);

Re: A researcher plans to identify each participant in a certain [#permalink]
24 Dec 2012, 00:28

1

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Bunuel wrote:

Practice: try to use the same concept.

Okay here goes,

The # of single letter codes possible would be n itself; The # of pair of distinct letters codes possible would be (in alphabetical order); nC2 The # of Triples of distinct letters codes possible would be (in alphabetical order); nC3

Re: A researcher plans to identify each participant in a certain [#permalink]
24 Dec 2012, 00:49

1

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Expert's post

eaakbari wrote:

Bunuel wrote:

Practice: try to use the same concept.

Okay here goes,

The # of single letter codes possible would be n itself; The # of pair of distinct letters codes possible would be (in alphabetical order); nC2 The # of Triples of distinct letters codes possible would be (in alphabetical order); nC3

Thus

nC3 + nC2 + n>12

n*(n-1)/2 + n*(n-1)*(n-2)/3*2 + n>12

Simplifying

n*(n^2 +5)>72

Only sufficient value of n = 4

Is it correct?

Correct.

Three letters A, B, and C, are enough for 7<12 codes: A; B; C; AB; AC; BC; ABC.

Four letters A, B, C, and D are enough for 15>12 codes: A; B; C; D; AB; AC; AD; BC; BD; CD; ABC; ABD; ACD; BCD; ABCD. _________________

Re: A researcher plans to identify each participant in a certain [#permalink]
13 Aug 2014, 23:54

1

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Expert's post

ccyang24 wrote:

Bunuel wrote:

sarb wrote:

A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetical order. What is the least number of letters that can be used if there are 12 participants, and each participant is to receive a different code?

A. 4 B. 5 C. 6 D. 7 E. 8

Say there are minimum of n letters needed, then;

The # of single letter codes possible would be n itself; The # of pair of distinct letters codes possible would be C^2_n (in alphabetical order);

Re: A researcher plans to identify each participant in a certain [#permalink]
03 Dec 2012, 01:30

Expert's post

ronr34 wrote:

Hi Bunnel

won't this C^2_n just give you all the pairs available? we need them also ordered....

Notice that we are told that letters in the code should be written in alphabetical order. Now, 2Cn gives different pairs of 2 letters possible out of n letters, but since codes should be written in one particular order (alphabetical), then for each pair there will be only one ordering possible, thus the number of codes out of n letters equals to number of pairs out of n letters.

Re: A researcher plans to identify each participant in a certain [#permalink]
23 Dec 2012, 12:22

Bunuel wrote:

sarb wrote:

A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetical order. What is the least number of letters that can be used if there are 12 participants, and each participant is to receive a different code?

A. 4 B. 5 C. 6 D. 7 E. 8

Say there are minimum of n letters needed, then;

The # of single letter codes possible would be n itself; The # of pair of distinct letters codes possible would be C^2_n (in alphabetical order);

Re: A researcher plans to identify each participant in a certain [#permalink]
22 Nov 2013, 14:15

Bunuel wrote:

sarb wrote:

A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetical order. What is the least number of letters that can be used if there are 12 participants, and each participant is to receive a different code?

A. 4 B. 5 C. 6 D. 7 E. 8

Say there are minimum of n letters needed, then;

The # of single letter codes possible would be n itself; The # of pair of distinct letters codes possible would be C^2_n (in alphabetical order);

Re: A researcher plans to identify each participant in a certain [#permalink]
22 Nov 2013, 14:19

Expert's post

yenpham9 wrote:

Bunuel wrote:

sarb wrote:

A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetical order. What is the least number of letters that can be used if there are 12 participants, and each participant is to receive a different code?

A. 4 B. 5 C. 6 D. 7 E. 8

Say there are minimum of n letters needed, then;

The # of single letter codes possible would be n itself; The # of pair of distinct letters codes possible would be C^2_n (in alphabetical order);

Re: A researcher plans to identify each participant in a certain [#permalink]
22 Nov 2013, 14:37

Bunuel wrote:

ronr34 wrote:

Hi Bunnel

won't this C^2_n just give you all the pairs available? we need them also ordered....

Notice that we are told that letters in the code should be written in alphabetical order. Now, 2Cn gives different pairs of 2 letters possible out of n letters, but since codes should be written in one particular order (alphabetical), then for each pair there will be only one ordering possible, thus the number of codes out of n letters equals to number of pairs out of n letters.

Hope it's clear.

Hi Bunuel,

From n letters we choose the number of pairs, the result will be C^2_n which may include 2 kinds of pairs (AB) and (BA). Still confused .

Re: A researcher plans to identify each participant in a certain [#permalink]
22 Nov 2013, 14:43

Expert's post

yenpham9 wrote:

Bunuel wrote:

ronr34 wrote:

Hi Bunnel

won't this C^2_n just give you all the pairs available? we need them also ordered....

Notice that we are told that letters in the code should be written in alphabetical order. Now, 2Cn gives different pairs of 2 letters possible out of n letters, but since codes should be written in one particular order (alphabetical), then for each pair there will be only one ordering possible, thus the number of codes out of n letters equals to number of pairs out of n letters.

Hope it's clear.

Hi Bunuel,

From n letters we choose the number of pairs, the result will be C^2_n which may include 2 kinds of pairs (AB) and (BA). Still confused .

Maybe the following example would help. Consider 4 letters {a, b, c, d}. How many 2-letter words in alphabetical order are possible? The answer is C^2_4=6: ab; ac; ad; bc; bd; cd. _________________

Re: A researcher plans to identify each participant in a certain [#permalink]
22 Nov 2013, 14:53

Hi Bunuel,

From n letters we choose the number of pairs, the result will be C^2_n which may include 2 kinds of pairs (AB) and (BA). Still confused .[/quote]

Maybe the following example would help. Consider 4 letters {a, b, c, d}. How many 2-letter words in alphabetical order are possible? The answer is C^2_4=6: ab; ac; ad; bc; bd; cd.[/quote][/quote]

Thanks a lot Bunuel. I got it now . Have a nice weekend!

gmatclubot

Re: A researcher plans to identify each participant in a certain
[#permalink]
22 Nov 2013, 14:53