She used 12 gallons this week and used 20% more than last week. That means 12 gallons is 120% of last weeks gas. Since 12 times 10 equals 120, then to find 20% you do 20 divided 10 and subtract the answer from 12.
20% is 2 because 20 divided by 10 is 2. 12 minus 2 is 10
