The events AA, BB and CC are such that P(A)=0.1\mathrm{P}(A) = 0.1, P(B)=0.2\mathrm{P}(B) = 0.2 and P(C)=c\mathrm{P}(C) = c. AA and BB are independent events. AA and CC are mutually exclusive events.

  1. (a)

    State the values of P(A∣B)\mathrm{P}(A | B) and P(A∣C)\mathrm{P}(A | C).

  2. (b)

    It is given that P(A∪B)=P(A′∩C′)\mathrm{P}(A \cup B) = \mathrm{P}(A' \cap C').

    Find the value of cc.

  3. (c)

    Find the maximum and minimum possible values of P(A′∩B′∩C′)\mathrm{P}(A' \cap B' \cap C').

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Worked solution

8(a)

1. State the value of $\mathrm{P}(A | B)$ using the property of independent events

Since AA and BB are independent events, the occurrence of BB does not affect the probability of AA.
P(A∣B)=P(A)=0.1\mathrm{P}(A | B) = \mathrm{P}(A) = 0.1

2. State the value of $\mathrm{P}(A | C)$ using the property of mutually exclusive events

Since AA and CC are mutually exclusive events, they cannot occur at the same time, meaning P(A∩C)=0\mathrm{P}(A \cap C) = 0.
P(A∣C)=P(A∩C)P(C)=0\mathrm{P}(A | C) = \frac{\mathrm{P}(A \cap C)}{\mathrm{P}(C)} = 0

Answer

P(A∣B)=0.1\mathrm{P}(A | B) = 0.1, P(A∣C)=0\mathrm{P}(A | C) = 0

8(b)

1. Calculate $\mathrm{P}(A \cup B)$ using the independence of $A$ and $B$.

Since AA and BB are independent, P(A∩B)=P(A)P(B)=0.1×0.2=0.02\mathrm{P}(A \cap B) = \mathrm{P}(A)\mathrm{P}(B) = 0.1 \times 0.2 = 0.02.
P(A∪B)=P(A)+P(B)−P(A∩B)\mathrm{P}(A \cup B) = \mathrm{P}(A) + \mathrm{P}(B) - \mathrm{P}(A \cap B)
P(A∪B)=0.1+0.2−0.02=0.28\mathrm{P}(A \cup B) = 0.1 + 0.2 - 0.02 = 0.28

2. Express $\mathrm{P}(A' \cap C')$ in terms of $c$ using the fact that $A$ and $C$ are mutually exclusive.

Since AA and CC are mutually exclusive, P(A∪C)=P(A)+P(C)=0.1+c\mathrm{P}(A \cup C) = \mathrm{P}(A) + \mathrm{P}(C) = 0.1 + c.
Using De Morgan's laws, P(A′∩C′)=1−P(A∪C)\mathrm{P}(A' \cap C') = 1 - \mathrm{P}(A \cup C).
P(A′∩C′)=1−(0.1+c)=0.9−c\mathrm{P}(A' \cap C') = 1 - (0.1 + c) = 0.9 - c

3. Equate the two probabilities and solve for $c$.

Equating P(A∪B)\mathrm{P}(A \cup B) to P(A′∩C′)\mathrm{P}(A' \cap C'):
0.28=0.9−c0.28 = 0.9 - c
c=0.9−0.28=0.62c = 0.9 - 0.28 = 0.62

Answer

c=0.62c = 0.62

8(c)

1. Express $\mathrm{P}(A' \cap B' \cap C')$ in terms of the unknown probability $x = \mathrm{P}(B \cap C)$.

From earlier parts, P(A∩B)=0.1×0.2=0.02\mathrm{P}(A \cap B) = 0.1 \times 0.2 = 0.02 and P(C)=c=0.62\mathrm{P}(C) = c = 0.62.
Let x=P(B∩C)x = \mathrm{P}(B \cap C). Since AA and CC are mutually exclusive, AA and CC do not overlap, meaning P(A∩C)=0\mathrm{P}(A \cap C) = 0 and P(A∩B∩C)=0\mathrm{P}(A \cap B \cap C) = 0.
By considering the separate, non-overlapping regions in a Venn diagram:
P(A only)=P(A)−P(A∩B)=0.1−0.02=0.08\mathrm{P}(A \text{ only}) = \mathrm{P}(A) - \mathrm{P}(A \cap B) = 0.1 - 0.02 = 0.08
P(B only)=P(B)−P(A∩B)−P(B∩C)=0.2−0.02−x=0.18−x\mathrm{P}(B \text{ only}) = \mathrm{P}(B) - \mathrm{P}(A \cap B) - \mathrm{P}(B \cap C) = 0.2 - 0.02 - x = 0.18 - x
P(C only)=P(C)−P(A∩C)−P(B∩C)=0.62−0−x=0.62−x\mathrm{P}(C \text{ only}) = \mathrm{P}(C) - \mathrm{P}(A \cap C) - \mathrm{P}(B \cap C) = 0.62 - 0 - x = 0.62 - x
The probability of the region entirely outside all three sets is:
P(A′∩B′∩C′)=1−[P(A only)+P(A∩B)+P(B only)+P(B∩C)+P(C only)]\mathrm{P}(A' \cap B' \cap C') = 1 - [\mathrm{P}(A \text{ only}) + \mathrm{P}(A \cap B) + \mathrm{P}(B \text{ only}) + \mathrm{P}(B \cap C) + \mathrm{P}(C \text{ only})]
P(A′∩B′∩C′)=1−[0.08+0.02+(0.18−x)+x+(0.62−x)]\mathrm{P}(A' \cap B' \cap C') = 1 - [0.08 + 0.02 + (0.18 - x) + x + (0.62 - x)]
P(A′∩B′∩C′)=1−(0.90−x)=0.10+x\mathrm{P}(A' \cap B' \cap C') = 1 - (0.90 - x) = 0.10 + x

2. Determine the valid range of values for $x = \mathrm{P}(B \cap C)$.

For the probabilities of the individual regions to be valid, they must be non-negative:
P(B∩C)=x≥0\mathrm{P}(B \cap C) = x \ge 0
P(B only)=0.18−x≥0  ⟹  x≤0.18\mathrm{P}(B \text{ only}) = 0.18 - x \ge 0 \implies x \le 0.18
P(C only)=0.62−x≥0  ⟹  x≤0.62\mathrm{P}(C \text{ only}) = 0.62 - x \ge 0 \implies x \le 0.62
Combining these conditions, the most restrictive bounds give the valid range for xx as:
0≤x≤0.180 \le x \le 0.18

3. Calculate the minimum and maximum possible values of $\mathrm{P}(A' \cap B' \cap C')$.

Substitute the minimum and maximum valid values of xx into the expression for P(A′∩B′∩C′)\mathrm{P}(A' \cap B' \cap C'):
Minimum P(A′∩B′∩C′)=0.10+0=0.1\text{Minimum } \mathrm{P}(A' \cap B' \cap C') = 0.10 + 0 = 0.1
Maximum P(A′∩B′∩C′)=0.10+0.18=0.28\text{Maximum } \mathrm{P}(A' \cap B' \cap C') = 0.10 + 0.18 = 0.28

Answer

Maximum =0.28= 0.28, Minimum =0.1= 0.1

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