Pack 5 kilograms, leaving 7 kilograms, pour 5 kilograms into a 9-kilogram bucket, and then pack 5 kilograms. Then the original barrel was only 2kg. First, add 2kg to one of the drivers' cars, pour all the remaining gasoline back into the 12kg barrel, fill up the 9Kg barrel, add 4kg to the 9Kg barrel, and then add the driver's car, and the driver will have 2+4 = 6kg, and give the rest to the other driver.