We want to travel 400 km. The average fuel consumption per 100 km is 5 liters. The price of 1 liter of fuel is 1.250 Eur.
So:
400 / 5 = 80
We will need 80 liters for 400 km.
80 * 1.250 = 100
The fuel cost for 400 km with a consumption of 5 l/100km is 100 Eur