It costs $36 for 12 gallons of gas. Mark is trying to determine how much it will cost to fill his 16-gallon tank. He divides 12 by 36 to find the unit rate and determines that it costs $0.33 per gallon. Then, he multiples $0.33 by 16 gallons to determine that the total cost to fill the tank is $5.33. What error did Mark make in calculating the cost to fill his car?