If the odds in favour of Boris beating Elena in a chess game are 5 to 4, what is the probability that Elena will win an upset victory in a best-of-five chess tournament?

----the answer is 7808/19683, i don't understand the chess thing basicly, i only can guess out what is the 19683, 3^9, but i don't know how to explain it. Can somebody explain me the chesses, and how to get the answer.

thanks a lot