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A local bank that has 15 branches uses a two-digit code to [#permalink]
27 Jul 2010, 11:25

00:00

A

B

C

D

E

Difficulty:

25% (low)

Question Stats:

67% (01:52) correct
32% (00:57) wrong based on 84 sessions

A local bank that has 15 branches uses a two-digit code to represent each of its branches. The same integer can be used for both digits of a code, and a pair of two-digit numbers that are the reverse of each other (such as 17 and 71) are considered as two separate codes. What is the fewest number of different integers required for the 15 codes?

A local bank that has 15 branches uses a two-digit code to represent each of its branches. The same integer can be used for both digits of a code, and a pair of two-digit numbers that are the reverse of each other (such as 17 and 71) are considered as two separate codes. What is the fewest number of different integers required for the 15 codes?

Choices A 3

B 4

C 5

D 6

E 7

Consider the code XY. If there are n digits available then X can take n values and Y can also take n values, thus from n digits we can form n^2 different 2-digit codes: this is the same as from 10 digits (0, 1, 2, 3, ..., 9) we can form 10^2=100 different 2-digit numbers (00, 01, 02, ..., 99).

We want # of codes possible from n digit to be at least 15 --> n^2\geq{15} --> n\geq4, hence min 4 digits are required.

I'm confused how xy would have n*n possibilities...wouldn't it b n*(n-1) possibilities because you would have to have two different digits? For example 17 and 71 are two different codes but 99 and 99 are the same code. Can someone explain?

I'm confused how xy would have n*n possibilities...wouldn't it b n*(n-1) possibilities because you would have to have two different digits? For example 17 and 71 are two different codes but 99 and 99 are the same code. Can someone explain?

It's always good to test theoretical thoughts on practice:

How many codes can be formed using 2 digits (n=2), 0 and 1.:

00; 01; 10; 11.

4=2^2.

Or consider the following: how many 2-digit codes can be formed out of 10 digits (0, 1, 2, 3, ..., 9)? 00; 01; 02; ... 99.

I'm confused how xy would have n*n possibilities...wouldn't it b n*(n-1) possibilities because you would have to have two different digits? For example 17 and 71 are two different codes but 99 and 99 are the same code. Can someone explain?

n * (n-1) is to say that 99 will not be chosen. To choose 99 once we are saying n*n should be the combo
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Re: Permutation question [#permalink]
31 Jan 2011, 16:58

Bunuel wrote:

zest4mba wrote:

A local bank that has 15 branches uses a two-digit code to represent each of its branches. The same integer can be used for both digits of a code, and a pair of two-digit numbers that are the reverse of each other (such as 17 and 71) are considered as two separate codes. What is the fewest number of different integers required for the 15 codes?

Choices A 3

B 4

C 5

D 6

E 7

Consider the code XY. If there are n digits available then X can take n values and Y can also take n values, thus from n digits we can form n^2 different 2-digit codes: this is the same as from 10 digits (0, 1, 2, 3, ..., 9) we can form 10^2=100 different 2-digit numbers (00, 01, 02, ..., 99).

We want # of codes possible from n digit to be at least 15 --> n^2\geq{15} --> n\geq4, hence min 4 digits are required.

Answer: B.

Hope it's clear.

Actually it could be A. B/c think of these arrangements for the 15 codes.

00, 01, 10, 02, 20, 03, 30, 11, 21, 12, 31, 13, 22, 23, 32 and 33. We have 16 arrangements so minimum # of different integers used can be 3.

What do you think?
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Re: Permutation question [#permalink]
31 Jan 2011, 17:03

Expert's post

mariyea wrote:

Bunuel wrote:

zest4mba wrote:

A local bank that has 15 branches uses a two-digit code to represent each of its branches. The same integer can be used for both digits of a code, and a pair of two-digit numbers that are the reverse of each other (such as 17 and 71) are considered as two separate codes. What is the fewest number of different integers required for the 15 codes?

Choices A 3

B 4

C 5

D 6

E 7

Consider the code XY. If there are n digits available then X can take n values and Y can also take n values, thus from n digits we can form n^2 different 2-digit codes: this is the same as from 10 digits (0, 1, 2, 3, ..., 9) we can form 10^2=100 different 2-digit numbers (00, 01, 02, ..., 99).

We want # of codes possible from n digit to be at least 15 --> n^2\geq{15} --> n\geq4, hence min 4 digits are required.

Answer: B.

Hope it's clear.

Actually it could be A. B/c think of these arrangements for the 15 codes.

00, 01, 10, 02, 20, 03, 30, 11, 21, 12, 31, 13, 22, 23, 32 and 33. We have 16 arrangements so minimum # of different integers used can be 3.

Re: Permutation question [#permalink]
02 Feb 2011, 09:22

Starting with choice A, 3 * 3 = 9 options are possible to code 15 branches. Not suff. Using 4 in choice B, 4 * 4 = 16 options are possible. We need the fewest. So B.

A local bank that has 15 branches uses a two-digit code to r [#permalink]
26 Mar 2013, 09:19

A local bank that has 15 branches uses a two-digit code to represent each of its branches. The same integer cannot be used for both digits of a code, are considered as two separate codes. What is the fewest number of different integers required for the 15 codes? Choices A 3

B 4

C 5

D 6

E 7
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Re: A local bank that has 15 branches uses a two-digit code to r [#permalink]
26 Mar 2013, 09:35

N= number of integers We have Noptions for the first one, N-1 options for the second. So a total of N(N-1) combinations, and we want that N(N-1)=15. Now or you plug in the different options, or you solve N^2-N-15=0; the first way seems faster, so lets try with A) 3 : 3*2=6 No B)4*3=12 No again C)5*4=20 YES

C
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Then I highlight the possible combinations (not considering the numbers with repeated integers) For example with 1 integer there are nor any number possible, with 2 integers, 2 possible numbers, with 3 integers , 6 possible numbers, with 4 integers, 12 possible numbers, and with 5 ... 20 . Correct Answer C

I know it´s not the finest answer, I guess it should be explained with combinatory.

Re: A local bank that has 15 branches uses a two-digit code to r [#permalink]
27 Mar 2013, 03:48

Expert's post

skamal7 wrote:

A local bank that has 15 branches uses a two-digit code to represent each of its branches. The same integer cannot be used for both digits of a code, are considered as two separate codes. What is the fewest number of different integers required for the 15 codes? Choices A 3

B 4

C 5

D 6

E 7

Merging similar topics. Please refer to the discussion above.

Re: A local bank that has 15 branches uses a two-digit code to [#permalink]
27 Mar 2013, 23:05

Let the required number of digits be n. Considering the given conditions, i)The first digit can be filled up in n ways. ii)The second digit can be filled up in n ways too. So we will get n * n = n^2 numbers.

n^2 \geq 15

=>n \geq 4 [since n is an integer.]

So,option B will be the correct answer. ------------------------------------------- Please press KUDOS if you like my post. _________________

Please press +1 KUDOS if you like my post.

gmatclubot

Re: A local bank that has 15 branches uses a two-digit code to
[#permalink]
27 Mar 2013, 23:05