Question

The sum of $15$ terms of an A.P. is $600$, and the common difference is $5$; find the first term.

Answer 1

Given $n=15,S=600$ and $d=5$

$S_n=\frac{n}{2}\{2a+(n-1\left)d\right\}600=\frac{15}{2}\{2a+(15-1\left)5\right\}80=2a+70\Rightarrow a=5$

So the first term is $5$