Well my car does 40mpg around town and it is 1.4.
With petrol at 1.20ppl around here and a UK Gallon is 4.54609, lets call it 4.55litres.
So 1.2*4.55 = £5.46/gallon/40 miles.
Thus, to do 20 miles, it will cost half of that and therefore total cost will be £2.23.
Simple maths guys...