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A person is to count 4500 notes. Let an 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 A. 24 minutes B. 34 minutes C. 125 minutes D. 135 minutes |
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Answer» Correct option B. 34 minutes 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 Sn = 3000 = n/2[2a + (n - 1)d] ⇒ n = 34 |
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