What will be the 21st term of the series given below?
3, 9, 15, ........
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 123
Each term is 6 more than the one before (3, 9, 15), so the series is an arithmetic progression, and its \(n\)th term is \(a+(n-1)d\): the first term plus the common difference added \(n-1\) times.
First term \(a=3\) and common difference \(d=9-3=6\)
The 21st term adds the difference 20 times: \(3+20\times6=123\)