Gasoline costs $2.79 per gallon is the number of gallons proportional to the total cost

I don't think it's proportional, because in order for a question to be a proportional question has/could be to be like this:

Gasoline costs $2.79 per 2 gallon. How much will it cost for 8 gallon. Then, we cross multiply and all to find the answer.

On monday,mrs.wise bought 3 lbs of apples at $4per lb= 4*3=$12on tuesday ,20%off and she buy 5 lbs of apples = $4-20%=3.20*5lb=$16example: other wise when she buy 5lbs on monday she have to spend 4*5=20