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Math Help - 7 doors

  1. #1
    Senior Member Paze's Avatar
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    7 doors

    I have 7 doors. I enter through one but have to exit through another.

    The solution I've come up with is 6!*7!. Would this be the correct solution? :/

    Thanks!
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  2. #2
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    Re: 7 doors

    Quote Originally Posted by Paze View Post
    I have 7 doors. I enter through one but have to exit through another.
    The solution I've come up with is 6!*7!. Would this be the correct solution?
    No it is not.
    It is simply 42 ways to enter and leave a room with 7 doors using different doors.
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  3. #3
    Senior Member Paze's Avatar
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    Re: 7 doors

    Quote Originally Posted by Plato View Post
    No it is not.
    It is simply 42 ways to enter and leave a room with 7 doors using different doors.
    Thanks. I am simply NOT getting probability :'(

    I find calculus much easier...
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  4. #4
    Senior Member Paze's Avatar
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    Re: 7 doors

    Quote Originally Posted by Plato View Post
    No it is not.
    It is simply 42 ways to enter and leave a room with 7 doors using different doors.
    Wait, Plato, let me rephrase my question.

    I have 7 doors and once I have entered one, I must exit through another. How many ways can I enter and exit through the doors?

    Is my answer now correct?

    I am translating these from Icelandic so sometimes I get some details wrong, unfortunately.
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    Re: 7 doors

    Don't be disheartened, Paze. Mathematics like this requires a lot of thought, as opposed to just a "plug-and-chug" approach seen in calculus courses. This sort of mathematics just requires A LOT of practice. You'll eventually get accustomed to the thought processing required by these sort of problems. Good luck with your endeavors.
    Last edited by Bashyboy; March 14th 2013 at 07:07 AM. Reason: Grammar
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    Re: 7 doors

    Quote Originally Posted by Paze View Post
    Wait, Plato, let me rephrase my question.

    I have 7 doors and once I have entered one, I must exit through another. How many ways can I enter and exit through the doors?

    Is my answer now correct?

    I am translating these from Icelandic so sometimes I get some details wrong, unfortunately.
    Problems like this rely heavily on the "multiplication property"- if event A can happen in n ways and B can happen, independently of A, in m ways then A and B can both happen in mn ways. You can enter through any of 7 doors so "entering" can happen in 7 ways. No matter what door you entered through there are then 6 doors you can leave through. There are 7(6)=42 ways to both enter and leave through different doors.
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  7. #7
    Senior Member Paze's Avatar
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    Re: 7 doors

    Thanks guys but what I mean is that I can enter again and again through the same door and exit through the row of 6 other options 6 times..That would give me 6!*7! right? I don't mean to enter and exit only once.
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    Re: 7 doors

    Hello, Paze!

    I have 7 doors. I enter through one, but have to exit through another.

    I assume that the question is:
    . . In how many ways can I pass through all the doors exactly once?


    Just talk your way through it . . .

    . . . . You have 7 choices of doors for entering.
    Then you have 6 choices of doors for exiting.
    Then you have 5 choices of doors for entering.
    Then you have 4 choices of doors for exiting.
    Then you have 3 choices of doors for entering.
    Then you have 2 choices of doors for exiting.
    Then you have 1 choice of doors for entering.

    Therefore . . .
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  9. #9
    Senior Member Paze's Avatar
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    Re: 7 doors

    Quote Originally Posted by Soroban View Post
    Hello, Paze!


    I assume that the question is:
    . . In how many ways can I pass through all the doors exactly once?


    Just talk your way through it . . .

    . . . . You have 7 choices of doors for entering.
    Then you have 6 choices of doors for exiting.
    Then you have 5 choices of doors for entering.
    Then you have 4 choices of doors for exiting.
    Then you have 3 choices of doors for entering.
    Then you have 2 choices of doors for exiting.
    Then you have 1 choice of doors for entering.

    Therefore . . .
    Well the question just asked for 42, e.g. how many times can I enter and exit but I expanded upon my initial view of the problem and I am now trying to confirm my suspicion.

    I am wondering: How many different ways of entering and exiting can I perform? I can enter through door A and exit through door B. I can enter through door A again and exit through door C or I could have entered through door A and exited through door B and then entered through door A again and exited through door B. These would be two different scenarios. How many scenarios can I make like this? I don't even understand my own question anymore. I'm trying to set it up but I fail to see it. I understand your solutions and thank you very much. I'm just having issues wrapping my head around probability, that's all.
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