Question:

A die is rolled. Consider events : \( A = \{1, 2, 5\} \), \( B = \{3, 5\} \), \( C = \{2, 3, 4, 5\} \). Find \( P(A \cap B|C) \) and \( P(A \cup B|C) \).

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Always perform the set operations (\( \cap \) or \( \cup \)) first to define the specific event, then calculate its intersection with the condition set \( C \).
Updated On: Sep 10, 2026
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Solution and Explanation

Concept:
• \( P(X|C) = \frac{n(X \cap C)}{n(C)} \).
• Set operations: Intersection \( \cap \) (common elements), Union \( \cup \) (all elements from both).

Step 1:
Find \( P(A \cap B|C) \)
First, find \( A \cap B = \{5\} \).
Now find the intersection with \( C \): \( (A \cap B) \cap C = \{5\} \cap \{2, 3, 4, 5\} = \{5\} \).
\[ P(A \cap B|C) = \frac{n((A \cap B) \cap C)}{n(C)} = \frac{1}{4} \]

Step 2:
Find \( P(A \cup B|C) \)
First, find \( A \cup B = \{1, 2, 3, 5\} \).
Now find the intersection with \( C \): \( (A \cup B) \cap C = \{1, 2, 3, 5\} \cap \{2, 3, 4, 5\} = \{2, 3, 5\} \).
\[ P(A \cup B|C) = \frac{n((A \cup B) \cap C)}{n(C)} = \frac{3}{4} \]
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