Answer
See below.
Work Step by Step
The nth term of an arithmetic sequence can be obtained by the formula $a_n=a_1+(n-1)d$ where $a_1$ is the first term and $d$ is the common difference.
Hence here $a_n=200+(n-1)(-20)$ and $a_{20}=200+(20-1)(-20)=200-380=-180$
