In the central city grand prix out of 30 cars that started the race,12 of them finished. what percent of the cars finished the race? what percent did not finished the race?

To get the answer we can use proportion.
30 100%
12 x
crossmultiply now
30x=12*100%
30x=1200%      /:30 (divide both sides by 30)
x=40% - its the percentage of cars which finished the race.
100%-40%=60% - its the answer

Subtract 12 from 30 so that you know how many DID NOT finish. 30-12=18. 
12/30 finished. Divide 12 by 30. 12÷30=0.4, and multiply by 100%. 0.4x100=40, so
40% finished.

18/30 did not finish. Divide 18 by 30.  18÷30=0.6, and multiply by 100%. 0.6x100=60, so
60% did not finish.