The method is the unitary method.
We know the price of 1 unit of a commodity.
We multiply it by the units purchased to get the total cost.
Here the price of each gallon of gas is $2.85
Mr Carrington purchased 12.8 gallons of gas.
Multiplying the quantity with the unit price, we get
The total amount = 12.8 × 2.85 = $36.48
The purchase of 12.8 gallons of gas shall cost $36.48.