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