Question

Find the sum of first $20$ multiples of $13$.

• A. $2,720$
• B. $2,730$
• C. $2,740$
• D. $2,750$

Answer 1

The correct option is B $2,730$

The multiples of $13$ are, $13,26,39,.....$
Therefore, $a=13,d=13,n=20$
We have $S_{20}=\frac{n}{2}[2a+(n-1\left)d\right]$
$=\frac{20}{2}[2\times 13+(20-1\left)13\right]$
$=10[26+19\times 13]$
$=10[26+247]$
Therefore, $S_{20}=2,730$