Aptitude Quizzes Mathematics Aptitude Test

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

Option A):
123
Option B):
121
Option C):
129
Option D):
117

Correct Answer is Option A):
123

Explanation:
Clearly, 3+6 = 9, 9+6 = 15 So, the series is an A.P. in which a = 3 .and d = 6. 2lst term a +(21-1)xd = a + 20xd = 3+ 20 x 6 = 123.