Multiple choice

21, 9, 21, 11, 21, 13, 21, ... What number should come next?

  1. 14

  2. 15

  3. 21

  4. 22

  5. 23
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

In this alternating repetition series, the random number 21 is interpolated every other number into an otherwise simple addition series that increases by 2, beginning with the number 9.

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