Answer
$S_{20}=1010$
Work Step by Step
RECALL:
The sum of the first $n$ terms ($S_n$) of an arithmetic sequence is given by the formula:
$S_n=\dfrac{n}{2}\left[2a+(n-1)d\right]$
where
$a$ = first term
$d$ = common difference
The given arithmetic sequence has:
$a=3
\\d=5$
Thus, to find the sum of the first 20 terms, substitute the given values to the formula above to obtain:
$S_n=\dfrac{n}{2}[2a+(n-1)d]
\\S_{20}=\dfrac{20}{2}[2(3) + (20-1)5]
\\S_{20}=10[6+19(5)]
\\S_{20}=10(6+95)
\\S_{20}=10(101)
\\S_{20}=1010$
