For number 5 you would divide the 90 miles by the two gallons of gasoline used which gives you 45. Your car is using one gallon of gas every 45 miles.
If your trip is going to be 270 miles long then you would divide the 270 by 45 to see how many gallons you would use. You would end up using 6 gallons.
Finally you would multiply the 6 by $2.80 to get $16.80
You would spend $16.80