CAT Quantitative Aptitude Questions | CAT Permutation and Combination Questions

CAT Questions | Permutation Probability | Selection

The question is from Permutation and Combination. This question is about Selection. We need to compute the probability of selecting a bag then a ball from it. This section hosts a number of questions which are on par with CAT questions in difficulty on CAT Permutation and Combination, and CAT Probability.

Question 29: A bag contains 4 red and 3 black balls. A second bag contains 2 red and 3 black balls. One bag is selected at random. If from the selected bag one ball is drawn, then what is the probability that the ball drawn is red?

  1. \\frac{39}{70}\\)
  2. \\frac{41}{70}\\)
  3. \\frac{29}{70}\\)
  4. \\frac{17}{35}\\)

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Explanatory Answer

Method of solving this CAT Question from Permutation and Combination: Clue - FInd the number of rearrangements and then focus on the number of digits

Let Red balls be 'r' and brown balls be 'b'
P(r) = p(b1) * p(r) + p(b2) * p(r)
P(R) = \\frac{1}{2}\\) * \\frac{4}{7}\\) + \\frac{1}{2}\\) * \\frac{2}{5}\\)
P(R) = \\frac{17}{35}\\)

The question is "If from the selected bag one ball is drawn, then what is the probability that the ball drawn is red?"

Hence the answer is "\\frac{17}{35}\\)"

Choice D is the correct answer.


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