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CA Foundation · Quantitative Aptitude · Sequence and Series

Evaluate the sum of Σ (2r + 3) for r = 1 to 20.

Split the sum into 2 times the sum of r plus 3 added twenty times. The sum of r from 1 to 20 is 210, so the total is 420 plus 60, which equals 480.

  1. A460Correct
  2. B520
  3. C440
  4. D480

Explanation

Σ(2r+3) = 2Σr + 3×20 = 2×(20×21/2) + 60 = 420 + 60 = 480. Check as AP: first term 5, last term 43, sum = 20×(5+43)/2 = 480. The value 460 comes from forgetting that the constant 3 is added 20 times and using 40 instead.

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