#### Answer

$840$ cars

#### Work Step by Step

We know the sequence is:
$20, 22, 24$ ...
We need to sum $21$ terms. Since this is an arithmetic sequence with $d=2$, we plug in:
$S_n=\frac{n}{2}[2a_1+(n-1)d]$
$S_{21}= \frac{21}{2}[2(20)+(21-1)(2)]=10.5*(40+40)=840$