Section B: Probability and Statistics [60 marks]
For events and , it is given that and .
- (a)
Find .
- (b)
It is also given that .
Find .
- (c)
For a third event , it is given that and are independent of each other, and and are mutually exclusive.
Find the maximum value of .
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Worked solution
5(a)
1. Express the probability of $B'$ as the sum of its mutually exclusive components involving $A$.
2. Substitute the given values to solve for $\mathrm{P}(A \cap B')$.
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,
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,
Since is the union of mutually exclusive events and , we have .
3. Substitute the known values to form an equation and solve for $\mathrm{P}(A)$.
Multiplying both sides by :
0.58
5(c)
1. Establish the bounds for $C$ based on its mutual exclusivity with $B$.
From parts (a) and (b), we know:
We are given .
Since and are mutually exclusive, . This means must lie entirely within , so is partitioned between and .
This restricts the intersections of :
2. Express the intersection probabilities using the independence of $A$ and $C$.
Since and are independent, it follows that and are also independent. We can rewrite the intersections in terms of :
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:
For to exist within both valid regions, must satisfy both inequalities, meaning it cannot exceed the smaller of the two bounds.
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