Question

Alice and her two sisters, Betty and Carol, are avid tennis players. Betty is the best of the three sisters, and Carol plays at the same level as Alice. Alice defeats Carol 50% of the time but only defeats Betty 40% of the time. Alice’s mother offers to give her $100 if she can win two consecutive games when playing three alternating games against her two sisters. Since the games will alternate, Alice has two possibilities for the sequence of opponents. One possibility is to play the first game against Betty, followed by a game with Carol, and then another game with Betty. We will refer to this sequence as BCB. The other possible sequence is CBC.
Calculate the probability of Alice getting the $100 reward if she chooses the sequence CBC.

Answer 1

Answer:

0.484

Step-by-step explanation:

Given that:

Alice defeats Carol 50% of the time.

This implies that, the probability of defeating Carol = 0.5

Also the probability of losing to Carol = 1 - 0.5 = 0.5

Similarly, Alice defeats Betty 40% of the time.

The probability of Alice defeating Betty = 0.4 and the  probability of losing to Betty = 1 - 0.4 = 0.6

However, in as much as Alice needs to win two consecutive games, the sequence which satisfies this condition are as follows:

Let W represent Win and L represent L.

Then the conditions are WWW, WWL and LWW

Since the outcome of each game is independent of the other games.

Then, the probability that Alice wins two games against Carol CBC is:

Pr(2W - CBC) = (0.5 × 0.4 × 0.5) + ( 0.5 × 0.4 × 0.5) + ( 0.5 × 0.4 × 0.5)

Pr(2W - CBC) = 0.1 + 0.1 + 0.1

Pr(2W - CBC) = 0.3

The probability that Alice wins two games against Betty (BCB) is:

Pr(2W - BCB) = (0.4 × 0.5 × 0.4) + (0.6 × 0.5 × 0.4) + ( 0.4 × 0.5 × 0.6)

Pr(2W - BCB) =  0.08 + 0.12 + 0.12

Pr(2W - BCB) = 0.32

However, the Probability of winning two consecutive games is the result of the addition of the probability of winning two consecutive games in the sequence CBC together with the probability of winning two consecutive games in the sequence of BCB.

i.e.

Pr(2W) = Pr(2W- CBC) + Pr(2W - BCB)

Pr(2W) = 0.3+0.32

Pr(2W) = 0.62

Finally, to calculate the probability that Alice will get $100 if she chooses the sequence CBC is:

$\dfrac{Pr(2W -CBC)}{Pr(2W)}= \dfrac{0.3}{0.62}$

$\mathbf{\dfrac{Pr(2W -CBC)}{Pr(2W)}= 0.484}$