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.
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