Lisa start her trip with 11 1/2 of gallons of gas in her cars gas tank she bought an additional 6 4/5 gallons on her trip in and arrived back home with 3 3/10 gallons left how much gas does she use on her trip

Quantity of gas in the tank with which Lisa started her journey = 11 1/2 gallons
                                                                                                    = 23/2 gallons
Quantity of gas additionally bought by Lisa during her trip = 6 4/5 gallons
                                                                                            = 34/5 gallons
Quantity of gas still left in the car after Lisa completed her trip = 3 3/10 gallons
                                                                                                   = 33/10 gallons
Then
Quantity of gas used by Lisa during her trip = [(23/2) + (34/5)] - (33/10) gallons
                                                                       = [(115 + 68)/10] - (33/10) gallons
                                                                       = (183/10) - (33/10) gallons
                                                                       = 150/10 gallons
                                                                       = 15 gallons
So we can see from the above deduction that Lisa used 15 gallons of gas during her trip. I hope the procedure is simple enough for you to understand.