Answer
$440$
Work Step by Step
Sum of first $n$th term of an arithmetic sequence can be calculated as:
$S_n=\dfrac{n}{2}[a_1+a_n]$
since, we have {$5,8,11,...., 50$}
This implies that $a_1=5,a_n=50,n=16$
Now,
$S_{16}=\dfrac{16}{2}[5+50]=440$