Question Number 152082 by Tawa11 last updated on 25/Aug/21
Answered by Ar Brandon last updated on 25/Aug/21
$$=\underset{\mathrm{1}\leqslant\mathrm{r}\leqslant\mathrm{80}} {\sum}\mathrm{r}=\mathrm{40}\left(\mathrm{81}\right)=\mathrm{3240} \\ $$
Commented by Tawa11 last updated on 26/Aug/21
$$\mathrm{Thanks}\:\mathrm{sir}.\:\mathrm{God}\:\mathrm{bless}\:\mathrm{you}. \\ $$