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.
