Answer
$S_{20}=1,470$
Work Step by Step
The sequence consisting of the first 20 positive multiples of 7 has $a_1=7$ and $d=7$. Using $S_n=\dfrac{n}{2}(2a_1+(n-1)d)$, then,
\begin{array}{l}
S_{20}=\dfrac{20}{2}(2\cdot7+(20-1)7)\\
S_{20}=10(14+133)\\
S_{20}=10(147)\\
S_{20}=1,470
.\end{array}
