Question

Mohan saves Rs. $1$ on the first day, Rs. $2$ on the second day, Rs. $3$ on the third day and so on. What will be his total saving after $40$ days?

Answer 1

Mohan's savings are Rs. $1$, Rs. $2$, Rs. $3$,....
Here, $t_1=1$, $n=40$
Therefore, $t_n=t_{40}=20$; $S_n=S_{40}=$?
$\Rightarrow S_n=\frac{n}{2}(t_1+t_n)$
$\Rightarrow S_{40}=\frac{40}{2}(1+40)$
$\Rightarrow S_{40}=\frac{40}{2}\times 41=820$
Hence, Mohan's total savings after $40$ days will be Rs. $820$.