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Multiple Choice Questions

A person is to count 4500 notes Let an denote the number of notes he counts in the nth minute I...........

Mathematics (NDA) Arithmetic progression

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A person is to count 4500 notes. Let a_n denote the number of notes he counts in the nth minute. If a₁ = a₂ = a₃ = ...... = a₁₀ = 150, and a₁₀, a₁₁, a₁₂, … are in AP with the common difference -2, then the time taken by him to count all the notes is-

24 minutes

34 minutes

125 minutes

135 minutes

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Explanation:

Total notes counted in first 10 minutes = 150×10 = 1500

Remaining notes = 3000

Further counting of notes form an A.P series with a = 150 , d = -2

S_n = 3000 = n / 2 [2a + (n- 1) d] ⇒ n = 34

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