We refueled for 50 Euros. The unit price of fuel is 1.250 Euros/liter. The car's consumption is 5 liters per 100 km.
So:
50 / 1.250 = 40
We refueled 40 liters
40 / 5 = 8 * 100 = 800
With the refueled fuel worth 50 Euros, we can travel approximately 800 km