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# Possible ways tp open the door (Permutation/Combination)

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Manager
Joined: 28 Jul 2004
Posts: 135
Location: Melbourne
Schools: Yale SOM, Tuck, Ross, IESE, HEC, Johnson, Booth
Possible ways tp open the door (Permutation/Combination)  [#permalink]

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12 Mar 2009, 17:07
A door can be opened only with a security code that consists of five
buttons: 1, 2, 3, 4, 5. If, to open the door you must press three codes, then how many possible ways are there to open the door? Assume that the same code may be repeated.

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kris

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Joined: 14 Aug 2007
Posts: 692
Re: Possible ways tp open the door (Permutation/Combination)  [#permalink]

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13 Mar 2009, 01:20
krishan wrote:
A door can be opened only with a security code that consists of five
buttons: 1, 2, 3, 4, 5.

I assume this means that each code consists of 5 digits. Then there are 5!= 120 codes possible.

krishan wrote:
If, to open the door you must press three codes, then how many possible ways are there to open the door? Assume that the same code may be repeated.

120*120*120 ways
Manager
Joined: 13 Mar 2009
Posts: 175
Re: Possible ways tp open the door (Permutation/Combination)  [#permalink]

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13 Mar 2009, 02:26
The number of possible ways to open the door
P (n, k) = $$\frac{n!}{(n - k)!}$$,
so P (5, 3) = $$\frac{5!}{2!} = 3 * 4 * 5 =$$ 60
Manager
Joined: 21 Feb 2009
Posts: 99
Re: Possible ways tp open the door (Permutation/Combination)  [#permalink]

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14 Mar 2009, 03:49
It should be 5!*5!*5! = 120 ways
Manager
Status: Stanford GSB
Joined: 02 Jun 2008
Posts: 89
Re: Possible ways tp open the door (Permutation/Combination)  [#permalink]

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14 Mar 2009, 04:45
As same codes can be repeated
Intern
Joined: 25 Dec 2008
Posts: 18
Schools: HBS, Stanford
Re: Possible ways tp open the door (Permutation/Combination)  [#permalink]

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14 Mar 2009, 09:03
botirvoy wrote:
sandipchowdhury wrote:
As same codes can be repeated

agree.

Disagree: you have 5! possibilities each time you type in a code (having the same code purely means you don't have to subtract duplicates which makes the question easier). Thus 5! x 5! x 5! is the correct answer.
Manager
Joined: 02 Mar 2009
Posts: 122
Re: Possible ways tp open the door (Permutation/Combination)  [#permalink]

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15 Mar 2009, 03:34
Ok now my take seems to be completely different on this one. Plz point out the flaw if any:

Since there are 5 numbers in a given code and there are 5 numbers, I think the total posssible codes are: 5*5*5*5*5 = 3125. No where in the question does it say that you are not allowed to repeat the same digit in the code.

Now since the codes can be repeated, 3125 is the number of code possible without combination. We will have to add this figure to the one possible with combination.

Now to select 3 codes out of 3125, we should use the combination formula. C(3125, 3). Now this yields a huge figure ( 3123*3124*3125)/6

This should be added to the prev figure, ie, total possibilities:
3125 + ( 3123*3124*3125)/6.

Now my concern is the magnitude of the solution. Rarely do I see such an insane answer.

Please provide the official answer and if someone can point out any flaw in the reasoning above, please let me know.
CEO
Joined: 17 Nov 2007
Posts: 3484
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
GMAT 1: 750 Q50 V40
Re: Possible ways tp open the door (Permutation/Combination)  [#permalink]

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15 Mar 2009, 03:48
krishan wrote:
A door can be opened only with a security code that consists of five
buttons: 1, 2, 3, 4, 5. If, to open the door you must press three codes, then how many possible ways are there to open the door? Assume that [color=#BF0000]the same code may be repeated[/color].

5 possibilities for each pressed button: 5*5*5=125 ways.
The problem is poorly written or I don't understand how to press three codes that consist of five buttons:
1) "a security code that consists of five buttons" - a code cannot consists of 5 buttons, can it?
2) "must press three codes" - maybe buttons?
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Manager
Joined: 01 Jan 2008
Posts: 217
Schools: Booth, Stern, Haas
Re: Possible ways tp open the door (Permutation/Combination)  [#permalink]

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15 Mar 2009, 04:54
May be one must press 3 buttons out of 5 simultaneously to open the door:)

So it is 5*4*3=60
Manager
Joined: 02 Mar 2009
Posts: 122
Re: Possible ways tp open the door (Permutation/Combination)  [#permalink]

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15 Mar 2009, 05:54
That makes it really easy and somewhat possible for the GMAT.

What I thought was:

1 code = 5 buttons
Therefore 3 codes = 15 Buttons.

Hence I gave that long explanation.

I am still unsure of its veracity but can someone tell me the flaw in that one? Also, OA plz

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

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Re: Possible ways tp open the door (Permutation/Combination) &nbs [#permalink] 15 Mar 2009, 05:54
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# Possible ways tp open the door (Permutation/Combination)

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