ProbabilityHardBloom L3

Question

$A,B,C$ are three events for which $P(A) = 0.4,P(B) = 0.6,P(C) = 0.5,P(A \cup B) = 0.75,P(A \cap C) = 0.35$ and $P(A \cap B \cap C) = 0.2$. If $P(A \cup B \cup C) \geq 0.75$, then $P(B \cap C)$ can take values

Options

A.0.15
B.0.2
C.0.3
D.0.4

Solution

Sol. [Key Concept: Use $P(A \cap \ B) = P(A) + P(B) - P(A \cup \ B)$ and bound $P(B \cap \ C)$.]

$$P(A \cap B) = P(A) + P(B) - P(A \cup B) = 0.25$$

$$\begin{matrix} & 0.75 \leq P(\text{ }A \cup \text{ }B \cup C) = P(\text{ }A) + P(\text{ }B) + P(C) - (P(\text{ }A \cap \text{ }B) \\ & \ + P(\text{ }B \cap C) + P(C \cap \text{ }A)) + P(\text{ }A \cap \text{ }B \cap C) \leq 1 \end{matrix}$$

$$\Rightarrow 0.1 \leq P(\text{ }B \cap C) \leq 0.35$$

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