# Math: Number of possible combinations when sorted alphabetically

I have 17 items:
ABCDEFGHIJKLMNOPQ
I am working on a project that combines PDF documents in order, and I need to know how many total options would be possible.

There can be any combination of them, providing each item can only appear in the list once and the results are always in alphabetical order.

For example these would all be valid,
ACB
ACE
BFJ

But these would not be valid:
BBB
BCA

How can the number of options be calculated?
LVL 16
###### Who is Participating?

Commented:
>> the results are always in alphabetical order.
>> ACB
Why is this allowed? Shouldn't it be ABC?

If you want options that are larger than 3 (or even 2, or just 1), then the total number of options is 2^17 less 1. To see why, take a binary word that is 17 bits long. If you select ABDE, then that could map to:
11011000000000000

To select all options, you would select all possibilities of 1's and 0's. There are 2^17 such possibilities. One of those possibilities is all 0's which means none are selected. That is why the total number of options is 2^17 less 1.
0

Commented:
Combinations without repetition (n=17, r=3)

List has 680 entries.
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Combinations and Permutations Calculator

http://www.mathsisfun.com/combinatorics/combinations-permutations-calculator.html

Thanks
JC
0

Commented:
17!/(17-3)!/3!
0

Commented:
Are you always selecting three?
If you can select any number, than the total possible combinations is
2^17 (which includes the case where you select none and the one where you select all)
So valid cases
[blank]
A
B
AB
ACDG
GHPQ
ABCDEFGHIJKLMNOPQ
etc

Oops phoffric beat me to the punch.
0

Author Commented:
Thank you-- 2 to the power of 17 minus one is the answer I was looking for.
0
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