I'm trying to calculate the number of potential options in creating characters for Dungeons and Dragons. Most of this is simple multiplication, but sometimes a character is allowed to choose two of something with no duplication and order doesn't matter.
Half-Elves can choose to increase two attributes from Strength, Dexterity, Constitution, Intelligence, and Wisdom. I thought the answer was number of initial choices times number of remaining choices (5x4=20), but that implies that it matters which one is selected first, which is irrelevant. I wrote it out, and discovered there are only 10 outcomes.
So then I thought the answer as number of initial choices times number of selections (5x2=10).
But then I increased the number of choices to six and wrote it out, and the answer is 15.
This is going to come up again with skills and weapons. So how do I calculate the number of potential combinations?