Question

Find sum of all odd integers between $2$ and $100$ divisible by $3$.

Answer 1

The list of required numbers is $3,9,15,21,...99.$

Thus, $a=3,d=6$ and $n=17$, $l=99$

Sum of all terms = $\frac{n(a+l)}{2}$

Sum $=\frac{17(3+99)}{2}=17\times 51=867$