New Enroll in RBI Grade B New Batch - Starts on May 21

    Question

    A bag contains 5 black balls,“a” white balls and

    “b” purple balls. If one ball taken out from the bag, then probability of being it white is 2/7, while the probability of being purple is 5/14. The sum between number of white and purple balls in that bag is?
    A 12 Correct Answer Incorrect Answer
    B 15 Correct Answer Incorrect Answer
    C 9 Correct Answer Incorrect Answer
    D 10 Correct Answer Incorrect Answer
    E none of these Correct Answer Incorrect Answer

    Solution

    Probability of white ball= a/(5 + a + b) a/(5 + a + b) = 2/7 5a – 2b = 10 …………(1) Probability of purple = b/(5 + a + b) = 5/14 9b – 5a = 25 ………….(2) From (1) and (2) we get  a = 4 b = 5 Therefore sum will be (a + b) = (4 + 5) = 9

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