|Don't ask to ask, just ask|
Here you could have explained why you can never remember between permutations, combinations, then maybe give a bit of math background regarding the number of permutations/combinations for n numbers and then explain how you came up with that answer
If you have the time; the need; and the arrogance to believe that it is your role to force your brand of fish down the OPs throat; feel free, have at it. I won't stop you.
But nor will I require it of you.
I prefer to believe that if the OP is actually interested in the math or its derivation, then having been shown that 'permutations' is the algorithm of choice for solving his problem, he will probably be perfectly able to go off and look up both the math and algorithms for himself. After all, he found this place to ask his question didn't he.
Alternatively, if he simply isn't interested, I haven't wasted my time coming up with my explanation -- which would non-authoritative, and probably imperfect -- only for him to ignore it; whilst others pick holes in it and I then have to defend it.
If the OP chooses to use my code, he will have to install Algorithm::Combinatorics; and that already contains a shitload of information and further reading that is far more authoritative and thorough than anything I have the time to construct.
With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)
In the absence of evidence, opinion is indistinguishable from prejudice.