We know that the tank is filled halfway, and it can hold 17.2 gallons, so we can just divide to figure out how much is half the tank (the amount we need to fill it)
17.2 / 2 = 8.6
We can check our work by multiplying
8.6 x 2 = 17.2
So it would take 8.6gal to fill it up. I hope this helps :)
