The events AA, BB and CC are such that P(A)=0.8P(A)=0.8, P(B)=0.2P(B)=0.2 and P(C)=0.6P(C)=0.6. It is also known that P(A∩B)=P(B)P(A\cap B)=P(B) and P(B∩C)=0.1P(B\cap C)=0.1.

  1. (a)

    Find exactly the maximum and minimum possible values of

    (i) P(A∩Bc∩C)P(A\cap B^{c}\cap C),

  2. (ii)

    P(A∩Bc∩C∣A∪C).P(A\cap B^{c}\cap C\mid A\cup C).

  3. (b)

    It is given further that AA and CC are independent, find the value of P(A∩Bc∩C).P(A\cap B^{c}\cap C).

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Worked solution

6(a)

1. Determine $P(A \cap B \cap C)$ using the given subset condition.

Since P(A∩B)=P(B)=0.2P(A \cap B) = P(B) = 0.2, the event BB must be a subset of AA (B⊆AB \subseteq A).
Therefore, the intersection of BB and CC is entirely contained within AA, meaning A∩B∩C=B∩CA \cap B \cap C = B \cap C.
P(A∩B∩C)=P(B∩C)=0.1P(A \cap B \cap C) = P(B \cap C) = 0.1

2. Express $P(A \cup C)$ in terms of $P(A \cap B^c \cap C)$.

We partition A∩CA \cap C into two mutually exclusive events based on BB:
P(A∩C)=P(A∩B∩C)+P(A∩Bc∩C)P(A \cap C) = P(A \cap B \cap C) + P(A \cap B^c \cap C)
P(A∩C)=0.1+P(A∩Bc∩C)P(A \cap C) = 0.1 + P(A \cap B^c \cap C)
Using the addition rule for probabilities:
P(A∪C)=P(A)+P(C)−P(A∩C)P(A \cup C) = P(A) + P(C) - P(A \cap C)
P(A∪C)=0.8+0.6−(0.1+P(A∩Bc∩C))P(A \cup C) = 0.8 + 0.6 - (0.1 + P(A \cap B^c \cap C))
P(A∪C)=1.3−P(A∩Bc∩C)P(A \cup C) = 1.3 - P(A \cap B^c \cap C)

3. Apply probability bounds to find the maximum and minimum values.

Since any probability must not exceed 11, we have P(A∪C)≤1P(A \cup C) \le 1:
1.3−P(A∩Bc∩C)≤11.3 - P(A \cap B^c \cap C) \le 1
P(A∩Bc∩C)≥0.3P(A \cap B^c \cap C) \ge 0.3
Also, since A∩C⊆CA \cap C \subseteq C, its probability is bounded by P(C)P(C):
P(A∩C)≤P(C)P(A \cap C) \le P(C)
0.1+P(A∩Bc∩C)≤0.60.1 + P(A \cap B^c \cap C) \le 0.6
P(A∩Bc∩C)≤0.5P(A \cap B^c \cap C) \le 0.5
Thus, the maximum possible value is 0.50.5 and the minimum possible value is 0.30.3.

Answer

Maximum =0.5,Minimum =0.3\text{Maximum } = 0.5, \quad \text{Minimum } = 0.3

6(ii)

1. Determine the required range of values for $P(A \cap B^c \cap C)$ from the previously given probabilities.

Let x=P(A∩Bc∩C)x = P(A \cap B^c \cap C).
Since P(A∩B)=P(B)=0.2P(A \cap B) = P(B) = 0.2, the event BB is a subset of AA, meaning A∩B∩C=B∩CA \cap B \cap C = B \cap C.
Therefore, P(A∩C)=P(A∩B∩C)+P(A∩Bc∩C)=0.1+xP(A \cap C) = P(A \cap B \cap C) + P(A \cap B^c \cap C) = 0.1 + x.
Using the addition rule, P(A∪C)=P(A)+P(C)−P(A∩C)=0.8+0.6−(0.1+x)=1.3−xP(A \cup C) = P(A) + P(C) - P(A \cap C) = 0.8 + 0.6 - (0.1 + x) = 1.3 - x.
Since any probability cannot exceed 11, P(A∪C)≤1  ⟹  1.3−x≤1  ⟹  x≥0.3P(A \cup C) \le 1 \implies 1.3 - x \le 1 \implies x \ge 0.3.
Additionally, since A∩Bc∩C⊆Bc∩CA \cap B^c \cap C \subseteq B^c \cap C, we have x≤P(C)−P(B∩C)=0.6−0.1=0.5x \le P(C) - P(B \cap C) = 0.6 - 0.1 = 0.5.
Thus, 0.3≤x≤0.50.3 \le x \le 0.5.

2. Express the required conditional probability in terms of $x$.

By the definition of conditional probability,
P(A∩Bc∩C∣A∪C)=P(A∩Bc∩C∩(A∪C))P(A∪C)P(A \cap B^c \cap C \mid A \cup C) = \frac{P(A \cap B^c \cap C \cap (A \cup C))}{P(A \cup C)}
Since A∩Bc∩CA \cap B^c \cap C is a subset of A∪CA \cup C, the intersection is simply A∩Bc∩CA \cap B^c \cap C, making the numerator P(A∩Bc∩C)=xP(A \cap B^c \cap C) = x.
From Step 1, we established that P(A∪C)=1.3−xP(A \cup C) = 1.3 - x.
Thus, the conditional probability expression simplifies to x1.3−x\frac{x}{1.3 - x}.

3. Find the maximum and minimum of the conditional probability using the feasible range of $x$.

We define the function f(x)=x1.3−x\mathrm{f}(x) = \frac{x}{1.3 - x} over the domain 0.3≤x≤0.50.3 \le x \le 0.5.
We can rewrite this algebraically:
x1.3−x=−(1.3−x)+1.31.3−x=−1+1.31.3−x\frac{x}{1.3 - x} = \frac{-(1.3 - x) + 1.3}{1.3 - x} = -1 + \frac{1.3}{1.3 - x}
As xx increases, the denominator (1.3−x)(1.3 - x) decreases, causing the fraction 1.31.3−x\frac{1.3}{1.3 - x} to increase. Hence, the function is strictly increasing over the interval [0.3,0.5][0.3, 0.5].
The minimum possible value occurs at x=0.3x = 0.3:
Minimum=0.31.3−0.3=0.31=310\text{Minimum} = \frac{0.3}{1.3 - 0.3} = \frac{0.3}{1} = \frac{3}{10}
The maximum possible value occurs at x=0.5x = 0.5:
Maximum=0.51.3−0.5=0.50.8=58\text{Maximum} = \frac{0.5}{1.3 - 0.5} = \frac{0.5}{0.8} = \frac{5}{8}

Answer

Maximum is 58\frac{5}{8}, minimum is 310\frac{3}{10}.

6(b)

1. Calculate the probability of the intersection of independent events $A$ and $C$.

Since AA and CC are independent,
P(A∩C)=P(A)P(C)=0.8×0.6=0.48P(A \cap C) = P(A)P(C) = 0.8 \times 0.6 = 0.48

2. Express $P(A \cap C)$ in terms of its disjoint subsets and solve for the required probability.

Given P(A∩B)=P(B)=0.2P(A \cap B) = P(B) = 0.2, we have P(A′∩B)=0P(A' \cap B) = 0.
This implies that P(A′∩B∩C)=0P(A' \cap B \cap C) = 0.
Since P(B∩C)=P(A∩B∩C)+P(A′∩B∩C)P(B \cap C) = P(A \cap B \cap C) + P(A' \cap B \cap C),
P(A∩B∩C)=0.1−0=0.1P(A \cap B \cap C) = 0.1 - 0 = 0.1
Now, decomposing P(A∩C)P(A \cap C) into disjoint subsets involving BB and BcB^c:
P(A∩C)=P(A∩B∩C)+P(A∩Bc∩C)P(A \cap C) = P(A \cap B \cap C) + P(A \cap B^c \cap C)
0.48=0.1+P(A∩Bc∩C)0.48 = 0.1 + P(A \cap B^c \cap C)
P(A∩Bc∩C)=0.48−0.1=0.38P(A \cap B^c \cap C) = 0.48 - 0.1 = 0.38

Answer

0.380.38

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