Question

Surendra 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 $20$ days?

Answer 1

Surendra's savings are Rs. $1$, Rs. $2$, Rs. $3$,.... which is an A.P.
Here, $t_1=1$, $n=20$
$t_n=t_{20}=20$; $S_n=S_{20}=$?

Therefore, $S_n=\frac{n}{2}(t_1+t_n)$

$\Rightarrow S_{20}=\frac{20}{2}(1+20)$

$\Rightarrow S_{20}=\frac{20}{2}\times 21=210$
Hence, Surendra's total savings after $20$ days will be Rs. $210$.