# 7 doors

• Mar 14th 2013, 06:39 AM
Paze
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!
• Mar 14th 2013, 06:46 AM
Plato
Re: 7 doors
Quote:

Originally Posted by Paze
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.
• Mar 14th 2013, 06:49 AM
Paze
Re: 7 doors
Quote:

Originally Posted by Plato
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...
• Mar 14th 2013, 06:52 AM
Paze
Re: 7 doors
Quote:

Originally Posted by Plato
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?

I am translating these from Icelandic so sometimes I get some details wrong, unfortunately.
• Mar 14th 2013, 06:56 AM
Bashyboy
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.
• Mar 14th 2013, 07:30 AM
HallsofIvy
Re: 7 doors
Quote:

Originally Posted by Paze
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?

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.
• Mar 19th 2013, 04:58 PM
Paze
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.
• Mar 19th 2013, 05:16 PM
Soroban
Re: 7 doors
Hello, Paze!

Quote:

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 . . .
• Mar 19th 2013, 05:36 PM
Paze
Re: 7 doors
Quote:

Originally Posted by Soroban
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.