Answer:
The probability that the series lasts exactly four games is ![3p(1-p)(p^{2} + (1 - p)^{2})](https://tex.z-dn.net/?f=3p%281-p%29%28p%5E%7B2%7D%20%2B%20%281%20-%20p%29%5E%7B2%7D%29)
Step-by-step explanation:
For each game, there are only two possible outcomes. Either team A wins, or team A loses. Games are played independently. This means that we use the binomial probability distribution to solve this question.
Binomial probability distribution
The binomial probability is the probability of exactly x successes on n repeated trials, and X can only have two outcomes.
![P(X = x) = C_{n,x}.p^{x}.(1-p)^{n-x}](https://tex.z-dn.net/?f=P%28X%20%3D%20x%29%20%3D%20C_%7Bn%2Cx%7D.p%5E%7Bx%7D.%281-p%29%5E%7Bn-x%7D)
In which
is the number of different combinations of x objects from a set of n elements, given by the following formula.
![C_{n,x} = \frac{n!}{x!(n-x)!}](https://tex.z-dn.net/?f=C_%7Bn%2Cx%7D%20%3D%20%5Cfrac%7Bn%21%7D%7Bx%21%28n-x%29%21%7D)
And p is the probability of X happening.
We also need to know a small concept of independent events.
Independent events:
If two events, A and B, are independent, we have that:
![P(A \cap B) = P(A)*P(B)](https://tex.z-dn.net/?f=P%28A%20%5Ccap%20B%29%20%3D%20P%28A%29%2AP%28B%29)
What is the probability that the series lasts exactly four games?
This happens if A wins in 4 games of B wins in 4 games.
Probability of A winning in exactly four games:
In the first two games, A must win 2 of them. Also, A must win the fourth game. So, two independent events:
Event A: A wins two of the first three games.
Event B: A wins the fourth game.
P(A):
A wins any game with probability p. 3 games, so n = 3. We have to find P(A) = P(X = 2).
![P(X = x) = C_{n,x}.p^{x}.(1-p)^{n-x}](https://tex.z-dn.net/?f=P%28X%20%3D%20x%29%20%3D%20C_%7Bn%2Cx%7D.p%5E%7Bx%7D.%281-p%29%5E%7Bn-x%7D)
![P(A) = P(X = 2) = C_{3,2}.p^{2}.(1-p)^{1} = 3p^{2}(1-p)](https://tex.z-dn.net/?f=P%28A%29%20%3D%20P%28X%20%3D%202%29%20%3D%20C_%7B3%2C2%7D.p%5E%7B2%7D.%281-p%29%5E%7B1%7D%20%3D%203p%5E%7B2%7D%281-p%29)
P(B):
The probability that A wins any game is p, so P(B) = p.
Probability that A wins in 4:
A and B are independent, so:
![P(A4) = P(A)*P(B) = 3p^{2}(1-p)*p = 3p^{3}(1-p)](https://tex.z-dn.net/?f=P%28A4%29%20%3D%20P%28A%29%2AP%28B%29%20%3D%203p%5E%7B2%7D%281-p%29%2Ap%20%3D%203p%5E%7B3%7D%281-p%29)
Probability of B winning in exactly four games:
In the first three games, A must win one and B must win 2. The fourth game must be won by 2. So
Event A: A wins one of the first three.
Event B: B wins the fourth game.
P(A)
P(X = 1).
![P(X = x) = C_{n,x}.p^{x}.(1-p)^{n-x}](https://tex.z-dn.net/?f=P%28X%20%3D%20x%29%20%3D%20C_%7Bn%2Cx%7D.p%5E%7Bx%7D.%281-p%29%5E%7Bn-x%7D)
![P(A) = P(X = 1) = C_{3,1}.p^{1}.(1-p)^{2} = 3p(1-p)^{2}](https://tex.z-dn.net/?f=P%28A%29%20%3D%20P%28X%20%3D%201%29%20%3D%20C_%7B3%2C1%7D.p%5E%7B1%7D.%281-p%29%5E%7B2%7D%20%3D%203p%281-p%29%5E%7B2%7D)
P(B)
B wins each game with probability 1 - p, do P(B) = 1 - p.
Probability that B wins in 4:
A and B are independent, so:
![P(B4) = P(A)*P(B) = 3p(1-p)^{2}*(1-p) = 3p(1-p)^{3}](https://tex.z-dn.net/?f=P%28B4%29%20%3D%20P%28A%29%2AP%28B%29%20%3D%203p%281-p%29%5E%7B2%7D%2A%281-p%29%20%3D%203p%281-p%29%5E%7B3%7D)
Probability that the series lasts exactly four games:
![p = P(A4) + P(B4) = 3p^{3}(1-p) + 3p(1-p)^{3} = 3p(1-p)(p^{2} + (1 - p)^{2})](https://tex.z-dn.net/?f=p%20%3D%20P%28A4%29%20%2B%20P%28B4%29%20%3D%203p%5E%7B3%7D%281-p%29%20%2B%203p%281-p%29%5E%7B3%7D%20%3D%203p%281-p%29%28p%5E%7B2%7D%20%2B%20%281%20-%20p%29%5E%7B2%7D%29)
The probability that the series lasts exactly four games is ![3p(1-p)(p^{2} + (1 - p)^{2})](https://tex.z-dn.net/?f=3p%281-p%29%28p%5E%7B2%7D%20%2B%20%281%20-%20p%29%5E%7B2%7D%29)