Multiple choice

Which number is wrong in the following number series. 3980, 3380, 2870, 2432, 2050, 1720, 1400

  1. 3380

  2. 3980

  3. 1400

  4. 1720

  5. 2050
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D Correct answer
Explanation

The series follows a pattern of subtracting decreasing multiples of 10: 3980 - 600 = 3380; 3380 - 510 = 2870; 2870 - 438 = 2432; 2432 - 382 = 2050; 2050 - 330 = 1720; 1720 - 320 = 1400. The pattern of differences is 600, 510, 438, 382, 330, 320. The sequence of differences of differences is 90, 72, 56, 52, 10. The term 1720 is incorrect because it breaks the progression of the differences.

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