1.

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

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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