Probability - Probability Bounds and Inequalities

Probability - Probability Bounds and Inequalities

9 marks

The events \(A\) and \(B\) satisfy \(P(A)=\dfrac{2}{5}\), \(P(B')=\dfrac{3}{4}\) and \(P(B\mid A)=\dfrac{1}{4}\).

  1. Find \(P(A'\cap B')\) and hence prove that the events \(A'\) and \(B'\) are independent.[4]

A third event \(C\) satisfies \(P(C)=\dfrac{1}{2}\) and \(P(A\cap B\cap C)=\dfrac{1}{20}\). It is also known that \(B\) and \(C\) are independent, while \(A\) and \(C\) are not independent.

  1. Find the range of possible values of \(P(A'\cap B'\cap C')\).[5]

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Answer:(i) \(P(A'\cap B')=\dfrac{9}{20}\); \(A'\) and \(B'\) are independent. (ii) \(\dfrac{3}{40}\le P(A'\cap B'\cap C')\le\dfrac{3}{8}\), with \(P(A'\cap B'\cap C')\ne\dfrac{9}{40}\).

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