The total amount of gasoline bought was given as 7 1/2 (or 7.5) gallons of gasoline. As you can see, I had to begin by converting the unit of gasoline to decimal and that way both variables (amount spent and gas purchased) will be in the same unit, which is decimals. You may convert the amount paid from decimal to fraction if you prefer, the important thing is getting both variables to be the same unit of measurement. Trying to solve a decimal and a fraction together would be unduly clumsy and would end up getting you confused.
The amount paid was 24.60 and the quantity purchased was 7.5!
To determine the cost per gallon of gasoline, you now divide the total amount spent by the amount of gallons bought.
Cost per gallon = 24.6/7.5
Cost per gallon = 3.28
Hence, at the amount she paid, the cost per gallon of gasoline is $3.28