Question

Find the sum of first seven numbers which are multiples of 2 as well as of 9.

Answer 1

Clearly, the first seven numbers which are multiples of 2, as well as 9, are $18,36,\mathrm{54.....126.}$

This is an AP with first term $a=18$ and last term $l=126$.

Let $S_n$ be the sum of n terms of an AP.

Then, $S_n=\frac{n}{2}(a+l)$
Putting $a=18,n=7$ and $l=126$ in $S_n$, we get
$S_7=\frac{7}{2}(18+126)\Rightarrow S_7=\frac{7}{2}\left(144\right)\Rightarrow S_7=504$