Meg started her trip with 11 1/2 gallons of gas in her car's tank. She bought an additional 6 4/5 gallons on her trip and arrived back home with 3 3/10 gallons left. How much gas did she use on the trip?

Meg used 15 gallons. First you find the lowest common denominator to add 11 1/2 and 6 4/5, which is 10. What you do to the bottom you have to do to the top. You multiply 2 and 5, which gives you 10. 1 times 5 is 5 ... 11 5/10. 2 times 5 is 10, 4 times 2 is 8 ... 6 8/10. Add 11 5/10 and 6 8/10 and it gives you 18 3/10. To find out how much she used subtract the total amount she bought and how much she returned home with. The fractions cancel out since 3/10 minus 3/10 equals 0. You're just left with 18 minus 3, which is 15.