To find what fraction of the cars are blue, we can take into account the total amount compared to how many are blue.
Seeing that there are 21 total cars, 18 of them being blue, we now have the fraction 18/21 to represent the number of blue cars. To find what fraction is blue, we simply reduce this number to the simplest form:
21 / 3 = 7
18 / 3 = 6
Hence, 6/7 of the 21 cars are blue.