I have a group of 20 different numbers that I'm trying to determine all
possible combinations for. The trick is that I need something that will
allow me to define the number of items in the combination pair.

For example, I need to be able to pull all possible combinations of 2
numbers within a group of 20 items. Only I need to be able to do this from 2
numbers up to 20.

I believe excel has the ability to do this, I am just unsure how to write
it. I do not need all permutations such as (1,2) and (2,1). Since I will be
taking the possible combinations and running a "sum" column down the side,
these two combinations would yield the same result and are, therefore,
unnecessary.