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A researcher palns to identify each participant in a certain

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Manager
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A researcher palns to identify each participant in a certain [#permalink] New post 10 May 2008, 22:36
A researcher palns 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
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Re: code [#permalink] New post 10 May 2008, 23:20
4 or 5
if order matters, it's 4

if order does not matter, it's 5
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Re: code [#permalink] New post 11 May 2008, 04:24
gmatnub wrote:
4 or 5
if order matters, it's 4

letters are to be arranged in alphabetical order so A,B can be arranged in one way AB..

gmatnub wrote:
if order does not matter, it's 5

if total n letters are used then total possible code
n (single letter code) + nC2 (double letter code)
now n + nC2 >=12
n + n(n-1)/2 > 12 giving least value of n = 5
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Re: code [#permalink] New post 11 May 2008, 04:35
OA is B

I didn't get why you divide n(n-1)/2?
Re: code   [#permalink] 11 May 2008, 04:35
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