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In "Number Series" question, a number series is given with one term missing. Choose the correct alternative that will same pattern and fill in the blank spaces

Look at this series: 21, 9, 21, 11, 21, 13, . . .
What number should come next?

Option:
A. 14
B. 15
C. 21
D. 23
Answer: C . 21

Justification:

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.

In this Series 21 is common random number.
Now add 2 with 9.
9 + 2 = 11
21 is next common number.
11 + 2 = 13
21 is next common number.
13 + 2 = 15.
21 is next common number.
So the answer is  c = 21.

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