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

Question: In the series 3, 9, 15, …… what will be the 21st term?
[A].

117

[B].

121

[C].

123

[D].

129

Answer: Option C

Explanation:

Clearly, 3 + 6 = 9, 9 + 6 = 15,…..

So, the series is an A.P. in which a = 3 and d = 6.

Therefore 21st term = a + (21 – 1) d = a + 20d = 3 + 20 x 6 = 123.