## College Algebra 7th Edition

$840$ cars
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: $S_n=\frac{n}{2}[2a_1+(n-1)d]$ $S_{21}= \frac{21}{2}[2(20)+(21-1)(2)]=10.5*(40+40)=840$