That's one of those math sequences where the answer is always known. No matter what digit you pick you would end up with either 81, 72, 63, 54, 45, 36, 27, 18, 9, or 0.
Whatever letter or symbol is associated with these numbers (and they are all the same) is the answer. Each time the board changes the associated letter is changed. All the other stuff is garbage to confuse you.