Question 5405
Use a proportionality:

{{{(40 cars)/(20 cars) = (what percent)/(100 percent)}}}
{{{40/20 = x/100}}}

cross multiply:
{{{20x=4000}}}
Divide both sides by 20
{{{x=200%}}}

But we want to know the amount of increase, so if 20 cars is 100%, then 40-20 cars = 200%-100%, or a 100% increase.