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NDA 2020
Exam
NDA
Year
2020
Subject
Mathematics
Chapter
Probability

NDA 2020 · Mathematics · Probability

If A and B are two events such that P(A)=0.6, P(B)=0.5 and P(A...

If AA and BB are two events such that P(A)=0.6,P(B)=0.5P(A)=0.6, P(B)=0.5 and P(A∩B)=0.4P(A \cap B)=0.4, then consider the following statements : 1. P(Aˉ∪B)=0.9\quad P(\bar{A} \cup B)=0.9. 2. P(Bˉ∣Aˉ)=0.6P(\bar{B} \mid \bar{A})=0.6. Which of the above statements is/are correct?

Options

  1. A1 only
  2. B2 only
  3. CBoth 1 and 2
  4. DNeither 1 nor 2

Correct answer

D. Neither 1 nor 2

Step-by-step solution

P(Aˉ)=1−P(A)=1−0.6=0.4P(Bˉ)=1−P(B)=1−0.5=0.5\begin{aligned} & P(\bar{A})=1-P(A)=1-0.6=0.4 \\ & P(\bar{B})=1-P(B)=1-0.5=0.5 \end{aligned} Now, P(A∪B)=P(A)+P(B)−P(A∩B)=0.6+0.5−0.4=0.7P(A \cup B)=P(A)+P(B)-P(A \cap B)=0.6+0.5-0.4=0.7 (1) Now P(Aˉ∪B)=P(Aˉ)+P(B)−P(Aˉ∩B)P(\bar{A} \cup B)=P(\bar{A})+P(B)-P(\bar{A} \cap B)=P(Aˉ)+P(B)−{P(B)−P(A∩B)}=0.4+0.5−{0.5−0.4}=0.8\begin{aligned} & =P(\bar{A})+P(B)-\{P(B)-P(A \cap B)\} \\ = & 0.4+0.5-\{0.5-0.4\}=0.8 \end{aligned}Hence,statement(1)isnotcorrect.Hence, statement (1) is not correct.\begin{aligned} & \text{ (2) } P(\bar{B} \mid \bar{A})=\frac{P(\bar{B} \cap \bar{A})}{P(\bar{A})}=\frac{P(B \cup A)^{\prime}}{P(\bar{A})}=\frac{1-P(A \cup B)}{P(\bar{A})} \\ & =\frac{1-0.7}{0.4}=0.75 . \end{aligned}$$ Hence, statement (2) is not correct.
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