Multiple choice

Directions: Complete the series.

1, 3, 9, 21, 41, ?

  1. 71

  2. 43

  3. 61

  4. 123

  5. 81
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A Correct answer
Explanation

From the series, it is clear that every term is added to 1 * 2, 2 * 3, 3 * 4..... as follows:

1 + 1 * 2 = 3, 3 + 2 * 3 = 9, 9 + 3 * 4 = 21, 21 + 4 * 5 = 41

Similarly, 41 + 5 * 6 = 71.

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