Answer :
Best way to start this is to figure out how much gas you can get for that $10. Since gas is $2.50/gallon and you buy $10 worth, that says you'll get 4 gallons of gas ($2.50 * 4 = $10). Now figure out how many gallons you have left in your car. If you car holds 13 gallons, 1/4 of that is 3.25 gallons. Add the 3.25 gallons left to the 4 gallons you bought and you get 7.25 gallons in your car.