Are you drinking the petrol, or putting it in a vehicle?
Assuming that you are putting it in a vehicle, the variable that makes the math is the average kilometers per liter (or miles per gallon).
The trip you are taking is about 20 kilometers. If you are driving a 2012 Jetta you would get someting like 13 km/liter. Take the trip length (20) and divide by the km/liter (13). 20/13 = 1.54 liters.
Hope that helps...