Step-by-step explanation:

To get the answer we can use proportion.
30----------100%
12-----------x
Cross multiply now
30x=12*100%
30x=1200% /:30 (divide both sides by 30)
x=40% - it's the percentage of cars which finished the race.
100%-40%=60% - it's the answer
Area of circle = πr^2
60=πr^2
we need to find radius (r)
so we solve for r
r^2=60/π
r=√(60/π)
r= 4.37