When increasing a percentage I usually just turn the percentage to a decimal. So, let's say you have 100 gallons of gas and it'll get an increase by 20%, just multiply 100 by .20 and you'll get 20, then add 20 to 100 to see what you'll have after. In this case, 15% would be .15, just multiply 22 by .15 to see how much the increase is, then add what you get to 22.