The first type: straight down 0.5 yuan/liter.
The second type: 6.69*0. 15= 1 yuan/liter, and the average decrease after two refuelings: 0.5 yuan/liter.
Example: Refueling100L
Type I:100 * (6.69-0.5) = 619 yuan, unit price: 6. 19 yuan/liter.
The second type:
First time: 100*6.69=669 yuan, original payment: 6.69 yuan/liter.
Second time:100 * 6.69-669 * 0.15 = 569 yuan, and the unit price after deduction is 5.69 yuan/liter.
The average unit price for two times is still: 6. 19 yuan/liter.
Generally speaking, gas stations take advantage and sell more oil.