Given :
The city mile driven to gallons of gas = 260 miles to 20 gallons
so, with 6 gallons , let the driven city miles = x
So,
driven miles : gallons of gas
260 : 20
x : 6
So, using the ratios to find x
so, he is correct
with 6 gallons of gas he would be able to drive 78 city miles.