To find out how much gas was remaining in the tank before Shaniyah filled it up, you can use the following formula:
Remaining gas = Total capacity of tank - Amount filled
Here, the total capacity of the tank is 120 gallons, and the amount filled is 102.85 gallons.
Remaining gas = 120 gallons - 102.85 gallons
Remaining gas = 17.15 gallons
So, the amount of gas remaining in the tank before she filled it up was 17.15 gallons.