JEE Main & Advanced Mathematics Question Bank Critical Thinking

  • question_answer
    One bag contains 5 white and 4 black balls. Another bag contains 7 white and 9 black balls. A ball is transferred from the first bag to the second and then a ball is drawn from second. The probability that the ball is white, is [DSSE 1987]

    A)                 \[\frac{8}{17}\]    

    B)                 \[\frac{40}{153}\]

    C)                 \[\frac{5}{9}\]       

    D)                 \[\frac{4}{9}\]

    Correct Answer: D

    Solution :

               Let a white ball be transferred from the first bag to the second. The Probability of selecting a white ball from the first bag \[=\frac{5}{9}.\]            Now the second bag has 8 white and 9 black. The probability of selecting white ball from the second bag\[=\frac{8}{17}\].            Hence required probability \[=\frac{5}{9}\times \frac{8}{17}=\frac{40}{153}\]            If a black ball be transferred from the first bag to the second, then the probability \[=\frac{4}{9}\times \frac{7}{17}=\frac{28}{153}\]                 Therefore required probability \[=\frac{40}{153}+\frac{28}{153}=\frac{4}{9}.\]


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