Answer
$S_{20}=1030$
Work Step by Step
The first n term of an arithmetic sequence can be calculated as:
$S_n=\frac{n}{2}(a_1+a_n)$
We have to find the 20th term first:
$a_{n}=a_1+d\times(n-1)$
$a_{20}=4+5\times(20-1)=4+95=99$
Therefore we can substitute in the formula of the sum, and we get:
$S_{20}=\frac{20}{2}(4+99))=10\times103=1030$
