Allow me to toss a question I've wondered about for a while into the plague of recent probability questions on this forum.
There's a magical die. Every single time you get a side, one of the dots gets rubbed off on that side. So, for example, if you're rolling the die for the first time and you roll a 5, that 5 will turn into a 4. And then your die will have 2 4s, 0 5s, and 1 of every other number. Then you roll a 1, the 1 magically turns into a 6, cuz that's just the way of the world. Then you have 2 6s, 2 4s, 0 5s, 0 1s, 1 2s and 1 3s.
So we have a pretty little cycle here. If you roll any side 6 times, you end up back where you started, as if nuthin' ever happened! For example, roll a 5 once and it turns into a 4. Then it turns into a 3, then a 2, then a 1, then a 6, then a 5 again!
Anyway, the question is this: If I roll the die 99 times, what is the probability every single side of the die will have the same number on it?
A friend proposed this to me a year ago. I've spent many hours on it, but I've honestly only made minimal progress - I'm simply too dumb to finish it. I've gotten an answer for 15, 21 and 27, and developed a basic understanding of the summations you'd have to do to get 99. 99 is crazy tho.
Anyway, no fancy computers (raw decimal form is overrated), and no looking shit up, or a vagina will grow from your penis and a penis will grow from your vagina. If you can't get the answer into closed form, just leave it with a shittonne of summa signs. It's nice to see how far you can get all by yourself.
P.S. The die is unbias :P