Q32 / 60 · JEE Advanced 2013 (Paper 2) · Mathematics

Paragraph: A box $B_{1}$ contains $1$ white ball, $3$ red balls and $2$ black balls. Another box $B_{2}$ contains $2$ white balls, $3$ red balls and $4$ black balls. A third box $B_{3}$ contains $3$ white balls, $4$ red balls and $5$ black balls.
Question: If $2$ balls are drawn (without replacement) from a randomly selected box and one of the balls is white and the other ball is red, the probability that these $2$ balls are drawn from box $B_{2}$ is

  • A
    1 1 6 1 8 1
  • B
    1 2 6 1 8 1
  • C
    6 5 1 8 1
  • Correct answer
    D
    5 5 1 8 1

Solution

Let A : one ball is white and other is red
E1 : both balls are from box B1
E2 : both ball are from box B2
E3 : both ball are from box B3
Hence, Prequired=PE2A
= P A E 2 · P E 2 P A E 1 · P E 1 + P A E 2 · P E 2 + P A E 3 · P E 3
= 2 C 1 × 3 C 1 9 C 2 × 1 3 1 C 1 × 3 C 1 6 C 2 × 1 3 + 2 C 1 × 3 C 1 9 C 2 × 1 3 + 3 C 1 × 4 C 1 1 2 C 2 × 1 3
= 1 6 1 5 + 1 6 + 2 1 1
= 5 5 1 8 1

Hence, the correct answer is option: 'D'.

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