Bob and Meena play a two-person game which is won by the first person to accumulate 10 points. At each turn Bob gains a point with probability of 1/3. If he doesn't get a point, then Meena gets a point. Meena is now ahead 9 to 8. What is the probability that Meena will win? Express your answer as a common fraction.
The probability that Meena wins is 21/25
In order for Meena to win, she needs to win the next turn or the following one, otherwise, she loses. The probability for that is equal to subtracting from 1 the probability of the complementary event: bob wins in the next 2 turns. Since each turn is independent of the other, we can obtain the probability of Bob winning the next 2 turns by taking the square of the probability of him winning on one turn, hence it is
Thus, the probability for Meena to win is 1-4/25 = 21/25.