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A bag contains 2 red, 3 white and 5 black balls, a ball is drawn its colour is noted and replaced. If the probability of getting a red ball for the first time at the n t h trial is atleast 2 25 , then the greatest value of n is equal to

Options

  1. A4
  2. B5
  3. C6
  4. D7

Correct answer

B. 5

Step-by-step solution

R → represents a red ball is drawn, P R = 2 10 = 1 5 Probability of getting red ball for the first time at n t h trial = 4 5 n - 1 1 5 ⇒ 4 5 n - 1 1 5 ≥ 2 25 ⇒ 4 5 n - 1 ≥ 2 5 n - 1 ≤ 4 ⇒ n ≤ 5

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