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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

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

In the Series 3, 9, 15, … what will be the 21st term ?

Option:
A. 117
B. 121
C. 123
D. 129
Answer: C . 123

Justification:

Clearly, 3 + 6 = 9,  9 + 6 = 15, ....
So, the series is an A.P. in which a = 3 and d = 6.
.'. 21st term = a + (21 -1)d = a + 20d = 3 + 20 * 6 = 123.

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