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.