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A person is to count 4500 currency notes. Let a_n denote the number of notes he counts in the n ^ th minute. If a ₁= a ₂= = a ₁₀=150 and a ₁₀, a ₁₁, are in A.P. with common difference -2 , then the time taken by him to count all notes is

Options

  1. A34 minutes
  2. B125 minutes
  3. C135 minutes
  4. D24 minutes

Correct answer

A. 34 minutes

Step-by-step solution

Till 10^ th minute number of counted notes =1500 3000= n 2 [2 148+(n-1)(-2)]=n[148-n+1] aligned & n^2-149 n+3000=0 & n=125,24 & n=125 is not possible. aligned Total time =24+10=34 minutes.

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