Section B: Probability and Statistics [60 marks]

For events AA and BB, it is given that P(B′)=0.51\mathrm{P}(B') = 0.51 and P(A′∩B′)=0.15\mathrm{P}(A' \cap B') = 0.15.
  1. (a)

    Find P(A∩B′)\mathrm{P}(A \cap B').

  2. (b)

    It is also given that P(B∣A)=1129\mathrm{P}(B \mid A) = \frac{11}{29}.

    Find P(A)\mathrm{P}(A).

  3. (c)

    For a third event CC, it is given that AA and CC are independent of each other, and BB and CC are mutually exclusive.

    Find the maximum value of P(C)\mathrm{P}(C).

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Worked solution

5(a)

1. Express the probability of $B'$ as the sum of its mutually exclusive components involving $A$.

P(B′)=P(A∩B′)+P(A′∩B′)\mathrm{P}(B') = \mathrm{P}(A \cap B') + \mathrm{P}(A' \cap B')

2. Substitute the given values to solve for $\mathrm{P}(A \cap B')$.

0.51=P(A∩B′)+0.150.51 = \mathrm{P}(A \cap B') + 0.15
P(A∩B′)=0.51−0.15=0.36\mathrm{P}(A \cap B') = 0.51 - 0.15 = 0.36

Answer

0.360.36

5(b)

1. Calculate $\mathrm{P}(A \cap B')$ from the given probabilities (which corresponds to solving part (a)).

From the definition of set differences in probability,
P(A∩B′)=P(B′)−P(A′∩B′)\mathrm{P}(A \cap B') = \mathrm{P}(B') - \mathrm{P}(A' \cap B')
=0.51−0.15=0.36= 0.51 - 0.15 = 0.36

2. Relate the conditional probability $\mathrm{P}(B \mid A)$ to $\mathrm{P}(A)$ and $\mathrm{P}(A \cap B')$.

Using the conditional probability formula,
P(B∣A)=P(A∩B)P(A)\mathrm{P}(B \mid A) = \frac{\mathrm{P}(A \cap B)}{\mathrm{P}(A)}
Since AA is the union of mutually exclusive events (A∩B)(A \cap B) and (A∩B′)(A \cap B'), we have P(A∩B)=P(A)−P(A∩B′)\mathrm{P}(A \cap B) = \mathrm{P}(A) - \mathrm{P}(A \cap B').
P(B∣A)=P(A)−P(A∩B′)P(A)\mathrm{P}(B \mid A) = \frac{\mathrm{P}(A) - \mathrm{P}(A \cap B')}{\mathrm{P}(A)}

3. Substitute the known values to form an equation and solve for $\mathrm{P}(A)$.

1129=P(A)−0.36P(A)\frac{11}{29} = \frac{\mathrm{P}(A) - 0.36}{\mathrm{P}(A)}
Multiplying both sides by 29P(A)29\mathrm{P}(A):
11P(A)=29(P(A)−0.36)11\mathrm{P}(A) = 29(\mathrm{P}(A) - 0.36)
11P(A)=29P(A)−10.4411\mathrm{P}(A) = 29\mathrm{P}(A) - 10.44
18P(A)=10.4418\mathrm{P}(A) = 10.44
P(A)=0.58\mathrm{P}(A) = 0.58

Answer

0.58

5(c)

1. Establish the bounds for $C$ based on its mutual exclusivity with $B$.

From parts (a) and (b), we know:
P(A∩B′)=0.36\mathrm{P}(A \cap B') = 0.36
P(A)=0.58  ⟹  P(A′)=1−0.58=0.42\mathrm{P}(A) = 0.58 \implies \mathrm{P}(A') = 1 - 0.58 = 0.42
We are given P(A′∩B′)=0.15\mathrm{P}(A' \cap B') = 0.15.

Since BB and CC are mutually exclusive, P(B∩C)=0\mathrm{P}(B \cap C) = 0. This means CC must lie entirely within B′B', so CC is partitioned between (A∩B′)(A \cap B') and (A′∩B′)(A' \cap B').
This restricts the intersections of CC:
P(A∩C)≤P(A∩B′)  ⟹  P(A∩C)≤0.36\mathrm{P}(A \cap C) \le \mathrm{P}(A \cap B') \implies \mathrm{P}(A \cap C) \le 0.36
P(A′∩C)≤P(A′∩B′)  ⟹  P(A′∩C)≤0.15\mathrm{P}(A' \cap C) \le \mathrm{P}(A' \cap B') \implies \mathrm{P}(A' \cap C) \le 0.15

2. Express the intersection probabilities using the independence of $A$ and $C$.

Since AA and CC are independent, it follows that A′A' and CC are also independent. We can rewrite the intersections in terms of P(C)\mathrm{P}(C):
P(A∩C)=P(A)P(C)=0.58P(C)\mathrm{P}(A \cap C) = \mathrm{P}(A)\mathrm{P}(C) = 0.58\mathrm{P}(C)
P(A′∩C)=P(A′)P(C)=0.42P(C)\mathrm{P}(A' \cap C) = \mathrm{P}(A')\mathrm{P}(C) = 0.42\mathrm{P}(C)

3. Solve the inequalities to determine the overall maximum value of $\mathrm{P}(C)$.

Substitute the expressions from Step 2 into the boundary limits from Step 1:

  1. 0.58P(C)≤0.36  ⟹  P(C)≤3658=1829≈0.6210.58\mathrm{P}(C) \le 0.36 \implies \mathrm{P}(C) \le \frac{36}{58} = \frac{18}{29} \approx 0.621
  2. 0.42P(C)≤0.15  ⟹  P(C)≤1542=514≈0.3570.42\mathrm{P}(C) \le 0.15 \implies \mathrm{P}(C) \le \frac{15}{42} = \frac{5}{14} \approx 0.357

For CC to exist within both valid regions, P(C)\mathrm{P}(C) must satisfy both inequalities, meaning it cannot exceed the smaller of the two bounds.

Maximum P(C)=514\text{Maximum } \mathrm{P}(C) = \frac{5}{14}

Answer

514\frac{5}{14}

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