How many terms are there in the A.P. 15, 21, 27, ..., 279?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 45
In an arithmetic progression the \(n\)th term is \(a+(n-1)d\). The last term 279 is set equal to it to find \(n\).
First term \(a=15\), common difference \(d=21-15=6\)
\(15+(n-1)\times6=279\)
\((n-1)\times6=264\)
\(n-1=44\), so \(n=45\)