The first car will travel 80 km in the first hour.
The second car will travel 100 km in the first hour.
The distance between them will be 20 km after the first hour.The second car will gain 20 km on the first car every hour. The distance between them will be 60 km when the second car gains 60 km on the first car.
This will take 3 hours.
So the answer is 3