JEE Main & Advanced JEE Main Paper (Held On 09-Jan-2019 Evening)

  • question_answer
    An urn contains 5 red and 2 green balls. A ball is drawn at random from the urn. If the drawn ball is green, then a red ball is added to the urn and if the drawn ball is red, then a green ball is added to the urn; the original ball is not returned to the urn. Now, a second ball is drawn at random from it. The probability that the second ball is red, is: [JEE Main Online Paper (Held On 09-Jan-2019 Evening]

    A) \[\frac{27}{49}\]                                            

    B)               \[\frac{26}{49}\]

    C)               \[\frac{32}{49}\]        

    D)               \[\frac{21}{49}\]

    Correct Answer: C

    Solution :

    5 Red and 2 green balls P(one red ball) \[=\,\,\frac{5}{7}\] P(one green ball) \[=\,\,\frac{2}{7}\] Case I: If drawn ball is green than a red ball is added \[\left( \begin{align}   & 6\,\,\operatorname{Re}d \\  & 1\,\,Green \\ \end{align} \right)P\,\,(red\,\,ball)\,\,=\,\,\frac{6}{7}\] Case II: If drawn ball is red then a green ball is added \[\left( \begin{align}   & 4\,\,\operatorname{Re}d \\  & 3\,\,Green \\ \end{align} \right)P\,\,(red\,\,ball)\,\,=\,\,\frac{4}{7}\] \[P({{2}^{nd}}\,red\,\,ball)\,\,=\,\,\frac{5}{7}\,\,\times \,\,\frac{4}{7}\,\,+\frac{2}{7}\,\,\times \frac{6}{7}=\frac{32}{49}\]                        


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