The chance of getting exactly eleven is $\frac {{20 \choose 11}{60 \choose 9}}{80 \choose 20}$ where the $20 \choose 11$ is the number of ways to pick the winning balls out of your $20$, the $60 \choose 9$ chooses the balls drawn that you did not pick, and the $80 \choose 20$ chooses the winning balls from all balls. You should be able to do higher numbers by analogy, then add them all up to get the total probability. Alpha finds\(60%20choose%2020-n\)%2F\(80%20choose%2020\)%20from%2011%20to%2020) a total probability of about $0.0008$, almost all of which comes from hitting $11$ or $12$.