I won't ruin it in here!
I explained it to one AW member, a while back, and I think his eyes walked out of his skull in protest of all the math.
I'll PM anyone who really wants to know. You have two options. One, I can just tell you HOW to come up with the right answer so you can look clever and then smile winningly at people who ask you how you did it, because the truth is you really have no clue how you did it. Or two, I can explain exactly why it works, and hence how to come up with the right answer. The second way is much math-ier and longer, but I think it's fun.