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A bag contains 10 white and 3 black balls. Balls are drawn one by one without replacement till all the black balls are drawn. The probability that the procedure of drawing balls will come to an end at the 7 t h draw is

Options

  1. A15 286
  2. B105 286
  3. C35 286
  4. D7 286

Correct answer

A. 15 286

Step-by-step solution

The drawing procedure ends at 7 t h draw, if 3 r d black ball comes at 7 t h draw. Hence, in 1 s t six draws there will be 2 black balls and 4 white balls. So, required Probability = C 4 10 × C 2 3 C 6 13 × 1 7 = 15 286

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