Answer
840
Work Step by Step
The number of cars that can park in a row form an arithmetic sequence
with first term $a=20$
and common difference $d=2$.
The number of cars that can park in the 21 rows is equal to
the sum of the first 21 terms of the arithmetic sequence.
$S_{n}=\displaystyle \frac{n}{2}[2a+(n-1)d]$
$S_{21}=\displaystyle \frac{21}{2}[2(20)+20(2)]$
$=10.5\cdot 80$
$=840$.
