JEE Main & Advanced Mathematics Probability Question Bank Odds in favour and odds against Addition theorem on probability

  • question_answer
    A coin is tossed twice. If events A and B are defined as :                 A = head on first toss,                                                                                                    \[B=\] head on second toss. Then the probability of                                                                                              \[A\cup B=\]

    A)                                                                                             \[\frac{1}{4}\]                                                                                           

    B)                                                                                             \[\frac{1}{2}\]

    C)                                                                                             \[\frac{1}{8}\]                                                                                           

    D)                                                                                             \[\frac{3}{4}\]

    Correct Answer: D

    Solution :

                       Total number of ways                                                                                     \[=(HH,\,HT,\,TH,\,TT)\]                                                                                                                         \[P\] (head on first toss)                                                                          \[=\frac{2}{4}=\frac{1}{2}=P(A)\]                                                                                                                         \[P\] (head on second toss)                                                                          \[=\frac{2}{4}=\frac{1}{2}=P(B)\]                                                                                                                         \[P\] (head on both toss)                                                                                \[=\frac{1}{4}=P(A\cap B)\]                    Hence required probability is,                                                            \[P(A\cup B)=P(A)+P(B)-P(A\cap B)=\frac{1}{2}+\frac{1}{2}-\frac{1}{4}=\frac{3}{4}\]  .


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