JEE Advanced · Mathematics · 32. Probability
Let \(X\) and \(Y\) be two events such that \(P(X \mid Y)=\frac{1}{2}\), \(P(Y / X)=\frac{1}{3}\) and \(P(X \cap Y)=\frac{1}{6}\). Which of the following is (are) correct?
- A \(P(X \cup Y)=\frac{2}{3}\)
- B \(X\) and \(Y\) are independent
- C \(X\) and \(Y\) are not independent
- D \(P\left(X^{c} \cap Y\right)=\frac{1}{3}\)
Answer & Solution
Correct Answer
(B) \(X\) and \(Y\) are independent
Step-by-step Solution
Detailed explanation
\(\because P(X / Y)=\frac{P(X \cap Y)}{P(Y)} \Rightarrow \frac{1}{2}=\frac{1 / 6}{P(Y)} \Rightarrow P(Y)=\frac{1}{3}\)
Similarly, \(\mathrm{P}(Y / X)=\frac{P(X \cap Y)}{P(X)}\)
\(\Rightarrow \frac{1}{3}=\frac{1 / 6}{P(X)} \Rightarrow P(X)=\frac{1}{2}\)
(a) \(P(X \cup Y)=P(X)+P(Y)-P(X \cap Y)=\frac{1}{2}+\frac{1}{3}-\frac{1}{6}=\frac{2}{3}\)
\(\therefore\) (a) is true.
(b) \(\because P(X \cap Y)=P(X) P(Y)\)
\(\Rightarrow X\) and \(Y\) are independent events.
\(\therefore\) (b) is true.
But (c) is not true.
(d) \(\mathrm{P}\left(X^{C} \cap Y\right)=P\left(X^{C}\right) \times P(Y)=\frac{1}{2} \times \frac{1}{3}=\frac{1}{6}\)
\(\therefore\) (d) is not true.
Similarly, \(\mathrm{P}(Y / X)=\frac{P(X \cap Y)}{P(X)}\)
\(\Rightarrow \frac{1}{3}=\frac{1 / 6}{P(X)} \Rightarrow P(X)=\frac{1}{2}\)
(a) \(P(X \cup Y)=P(X)+P(Y)-P(X \cap Y)=\frac{1}{2}+\frac{1}{3}-\frac{1}{6}=\frac{2}{3}\)
\(\therefore\) (a) is true.
(b) \(\because P(X \cap Y)=P(X) P(Y)\)
\(\Rightarrow X\) and \(Y\) are independent events.
\(\therefore\) (b) is true.
But (c) is not true.
(d) \(\mathrm{P}\left(X^{C} \cap Y\right)=P\left(X^{C}\right) \times P(Y)=\frac{1}{2} \times \frac{1}{3}=\frac{1}{6}\)
\(\therefore\) (d) is not true.
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