## Precalculus (6th Edition) Blitzer

$-180$
We need to use the formula $a_n =a_1+(n-1) d$ Here, $a_1=200$ and $d=-20$ Plug $n=20$ $a_{20}=200 +(20-1) \times (-20)$ or, $=200-380$ or, $a_{20}=-180$