Been done before, sometimes it's a random number, sometimes it's how many times you have sex in a week, your birthday, whatever. Then they just do extraneous problems to get you to think it's a big mystery, then bang, there is your number again.
Someone explained the way they do it in detail on here before but I can't for the life of me remember. I just know it's a relatively simple equation with a bunch of extra ones that cancel out.